CHAPTER 8 TEST EQUIPMENT

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1 CHAPTER 8 TEST EQUIPMENT The operational theory of equipment is only one part of the knowledges you need to maintain avionics equipment. You also need a knowledge of avionics drawings, schematics, and test equipment. You use many publications to properly maintain a weapons system in modern-day aircraft because they are so complex. Just the list of the electronics equipment installed in modern-day aircraft is lengthy. It is impossible for each individual to know all the various types of electronics equipment presently in use. However, with a good general background on electronic principles and circuit theory and a little study, you, the Aviation Electronics Technician, can rapidly become familiar with any specific system or test equipment. In this chapter, you will learn about some common test equipment used by Aviation Electronic Technicians (ATs). This information is in addition to modules 3 and 16 of the Navy Electronic and Electricity Training Series (NEETS) on test equipments. Review and refer to the NEETS modules as necessary for additional information about the test equipment described in this chapter. No in-depth theory beyond that necessary to describe the operation of the test set under discussion is included here. When you use a piece of test equipment with which you are not familiar, always use the appropriate instruction manual. These publications contain detailed and specific information about the particular equipment. CARE AND USE OF AVIONICS SUPPORT EQUIPMENT Learning Objective: Identify the proper care and use of avionics support equipment to include calibration, repair, and handling requirements. All electronic maintenance shops have and require many pieces of test equipment to maintain different types of electronic units. However, there are very few spare test sets. When a test set becomes inoperative, shop maintenance suffers. Therefore, each person should use the test equipment properly and only for its designed purpose. Protect the equipment from physical harm that may result from dropping, falling, or any other careless misuse, and always observe proper operating techniques. One of the chief causes of test set failure is carelessness. The user can be careless in an operating procedure or in handling the set. Improper range selection for the measured quantity is the most common mistake in an operating procedure. Such an error might be to try to measure 250 volts on the 50-volt scale of a meter. If you aren t sure about proper use of a test set, refer to the manual issued with the set. Improper handling causes damage to test equipment. Often, technicians place test sets near the edge of the bench where they can be easily knocked or pulled off. Read the instructions for proper handling and operating procedures, and think when you use a piece of equipment. Refer to NEETS, modules 3 and 16, for further information on test equipment operation and theory. CALIBRATION Test sets require checks to determine if they are within operating tolerances. Some test sets are used as frequency standards and require periodic calibration. You should always follow the recommendations of the manual or pamphlet issued with the set, unless current instructions change those recommendations. Normally, personnel in an intermediate-level maintenance shop perform calibration using special-purpose calibration equipment. Personnel at the organizational level of maintenance seldom calibrate test equipment. 8-1

2 REPAIR The using activity normally makes any minor repair of test sets not requiring calibration. Repairs are usually limited to the replacement of test leads and fuses. Before you make any repair, consult current instructions on repair of test equipment. Personnel assigned to an intermediate-level maintenance activity repair test equipment on a wider scale. Repair can vary from the replacement of circuit components to modules, depending on the authorized level of repair. However, most test equipment work at this level consists of calibrating equipment. HANDLING PRECAUTIONS Some equipments require special handling; however, several precautions apply to test equipments in general. Rough handling, moisture, and dust all affect the useful life of test equipment. For example, bumping or dropping a test instrument can destroy the calibration of a meter or short circuit the elements of an electronic tube within the instrument. Creasing or denting coaxial test cables alter their attenuating effect, affecting the accuracy of any RF measurements made with these cables. To reduce the danger of corrosion to untreated parts, always store test equipment in a dry place when not in use. Excessive dust and grime inside a test equipment affect its accuracy. Be sure all assembly screws that hold the case of the test equipment in place are tight and secure. As an added precaution, place all dust covers on test equipments when they are not in use. Meters are the most delicate part of test equipments. To make sure the meter maintains its accuracy, you should follow these additional precautions: Make certain the amplitude of the input signal under test is within the range of the meter. Keep meters as far away as possible from strong magnets. When servicing an item of electronic equipment that contains a meter, disconnect the meter from the circuit before making resistance or continuity tests. This precaution should prevent the possibility of burning out the meter. The instruction manuals that come with a piece of equipment contain the procedures for properly stowing test equipment cables and other accessories. Read these manuals carefully and follow the equipment instructions. Improper stowage of accessories could change cable characteristics and cause intermittent shorts in cables and leads. Improper stowage causes unreliable test equipment indications. Q1. Q2. Q3. Q4. Name one of the chief causes of test set failure. Although test equipment is repaired at the intermediate-level maintenance activity, most work performed at this level on test equipment consists of What is the most delicate part of a piece of test equipment? List the basic measuring parameters of electronic equipment. MEASURING INSTRUMENTS Learning Objective: Recognize types and uses of measuring equipment to include electronic meters, frequency measurement, and power measurement. In this chapter, the term measuring instruments includes only the class of test equipments that measure the basic parameters of an electronic equipment. The basic parameters are voltage, current, resistance, power, and frequency. METER OPERATION There must be some source of power available to operate a meter. Some meters use batteries installed in the meter case as a power source; others may use an electrical power cord plugged into a power receptacle. A vacuum tube voltmeter (VTVM) is an example of the second type. The power to operate some meters (such as meggers) is self-produced by manual operation of a handcrank. Most meters provide the means to measure more than one electrical quantity; these are multimeter. Before discussing any one particular type of meter, a brief review of each of the basic meters is necessary. For more details refer to NEETS, modules 3 and

3 Ammeter The amplitude of current flow through the basic meter mechanism limits it to measuring a fixed range of only a fraction of an ampere. A current shunt overcomes this limitation and protects the mechanism. The current shunt is actually a resistance of low value, permitting the instrument to serve as a dc ammeter that can measure relatively large direct currents. The current distribution between meter movement and shunt is inversely proportional to their individual resistances. Thus, the shunt, which has less resistance, carries most of the current. Since the meter coil carries only a small portion of the circuit current, it can indicate relatively large values of circuit current. The instrument provides a variety of current ranges by the use of shunts of different values. Figure 8-1 shows a simplified schematic diagram of an ammeter section taken from a typical volt-ohm-milliammeter (VOM). Ohmmeter The midscale deflection of an ohmmeter occurs when the current drawn by the meter is one-half the value of the current at full-scale (zero ohms) deflection. This condition exists when the measured resistance is equal to the total meter circuit resistance. Analysis of the circuit in figure 8-2 shows that full-scale deflection occurs when shorting the meter probes together. Less than full-scale deflection occurs when the resistance to be measured, Rx, is connected into the circuit. If the meter now reads one-half of its former current, the total circuit resistance Figure 8-2.-Series-type ohmmeter basic circuit. has doubled. This indicates that RX is equal to the total meter circuit resistance. Since the ohms-calibrated scale is nonlinear, the midscale portion represents the most accurate portion of the scale. The usable range extends with reasonable accuracy on the high end to 10 times the midscale reading. However, on the low end it decreases to one-tenth of the midscale reading. To extend the range of an ohmmeter, the proper values of shunt and series resistors and battery voltages are connected into the circuit. The proper values let you read the meter full scale with the test leads shorted. Figure 8-3 shows a Figure 8-1.-Simplified schematic diagram of an ammeter. Figure 8-3.-Simplified schematic diagram of an ohmmeter. 8-3

4 simplified schematic diagram of an ohmmeter section taken from a typical VOM. Voltmeter Adding a voltage-multiplying resistor makes the basic meter mechanism suitable for use when measuring dc voltages. The voltage-multiplying resistor is placed in series with the coil (fig. 8-4) and limits the flow of current to a safe value. Since the value of the resistor is constant for any given application, the flow of current through the coil is proportional to the voltage under measurement. By properly calibrating the dial, the instrument indicates voltage. However, it is actually the current that activates the meter. The use of different values of multiplying resistors establishes the voltage ranges of the instrument. MULTIMETER Much of the work that you do using a VOM can be done with a multimeter. The name multimeter comes from multiple meter, which is exactly what a multimeter is. It is an ohmmeter, a dc and an ac milliammeter, and a voltmeter. A typical multimeter is shown in figure 8-5. Figure 8-5.-Typical multimeter. Figure 8-4.-Simplified schematic diagram of a dc voltmeter. In many shops, you might use a portable, battery-operated multimeter such as a TS-352, USM-311, Simpson 260, or Simpson 160 for field use (troubleshooting in the aircraft, for instance). As an AT, however, you will often need a more sensitive meter one that gives more accurate readings and has wider ranges. Often, equipment schematics and wiring diagrams specify that voltages indicated at test points were obtained with a meter of a certain sensitivity, such as a 20,000-ohms-per-volt meter. You should use a meter with the same sensitivity 8-4

5 in repairing that equipment to obtain accurate readings because of circuit loading. NOTE: For a review of the basic theory and operation of the multimeter, refer to NEETS, module 3. MILLIOHMMETER One of the most common and troublesome problems is finding the exact location of a short circuit in a power distribution circuit involving many parallel paths. This and several troubleshooting problems are easier to solve with a milliohmmeter. A milliohmmeter is a low-range ohmmeter that can measure resistances in the milliohm range or less. The AN/USM-21A is a typical milliohmmeter used in the fleet. It can measure resistances in the range of 10 milliohms or less. Most ohmmeters read zero at such a low value. When using a milliohmmeter, you may encounter several problems. These problems include stray circuit resistances, such as contact resistance, test lead resistance, and switching resistance. In the conventional low-range ohmmeters, the primary problem is in the contact resistance at the test probes. The design of the AN/USM-21A overcomes the contact resistance problem. MEGOHMMETER (MEGGER) The megohmmeter, commonly called the megger, is an instrument that applies a high voltage to the component under test and measures the current leakage of the insulation. This lets you check a capacitor or an insulated cable for leakage under much higher voltages than an ohmmeter can supply. The megger consists of a hand-driven dc generator and an indicating meter. It measures resistances of many megohms. There are various resistance ratings of meggers with full-scale values as low as 5 megohms and as high as 10,000 megohms. Figure 8-6 shows the scale of a 100-megohm, 500-volt megger. Notice that the upper limit is infinity and that the upper end of the scale is also crowded. The first scale marking below infinity represents the highest accurate value the instrument can provide. Thus, if the pointer goes to infinity while you are making a test, it means that the resistance is higher than the range of the set. There are also various voltage ratings of meggers, such as 100, 500, 750, 1,000, and 2,500. The most common type is the one with a 500-volt rating. This voltage rating refers to the maximum output voltage of the megger. The output voltage depends on the turning speed of the crank and armature. When the megger s armature rotation reaches a predetermined speed, a slip clutch maintains the armature at a constant speed. The voltage rating is important. If too high a voltage is applied, it will cause even a good component to break down. Therefore, do not use a 500-volt megger to test a capacitor rated at 100 volts. You can use meggers to test the insulation resistance of conductors that may be shorting or breaking down under high voltage. In some situations, you can use meggers in the prevention of unnecessary breakdowns. You could maintain a record of insulation resistance of power and high-voltage cables, motor and generator windings, and transmission lines. These records reflect fluctuations in resistance and help Figure 8-6.-Scale of a 100-megohm, 500-volt megger. 8-5

6 determine when to replace the components to prevent a breakdown. Meggers are used for testing capacitors whose peak voltages are not below the output of the megger. They are also used for testing for high-resistance grounds or leakage on devices such as antennas and insulators. The following are precautions you should take when using meggers: 1. When you are making a megger test, do not energize the equipment. Disconnect it entirely from the system before testing. 2. Observe all safety rules in preparing equipment for test and in testing, especially when testing installed high-voltage apparatus. 3. Use well-insulated test leads, especially when using high-range meggers. Check the leads after connecting them to the megger and before connecting them to the component under test. Operate the megger and make sure there is no leak between the leads. The reading should be infinity. Check the leads by touching the test ends of the leads together while turning the crank slowly. The reading should be about zero. If the indication reads differently, you may have a faulty lead or a loose connection. 4. When using high-range meggers, take proper precautions against electric shock. There is enough capacitance in most electrical equipment to store up energy from the megger generator to give a very disagreeable and even dangerous electric shock. Because there is a high protective resistance in the megger, its open circuit voltage is not as dangerous as it would otherwise be; still, be careful. 5. Discharge equipment having considerable capacitance before and after megger tests. This should help you avoid receiving a dangerous shock. You can do this by grounding or short circuiting the terminals of the equipment under test. The AN/PSM-25, shown in figure 8-7, is a common megger used through the fleet. For more information on meggers, refer to NEETS, module 16. ELECTRONIC METERS Electronic meters and nonelectronic meters are used for the same purposes; however, they do have some differences. In the electronic multimeter and corresponding nonelectric measuring devices, the current- and resistance-measuring Figure 8-7.-AN/PMS-25 megger. circuits function in the same way. However, when an electronic multimeter is used to measure voltage, an amplifier is involved. Therefore, the electronic meter requires calibration before it is used. The proper calibration and use of the instruments vary slightly, according to model. You should refer to the operation instruction manual for the specific details of each model. The ordinary voltmeter cannot be accurately used to make voltage measurements in highimpedance circuits. For example, you need to measure the plate voltage of a pentode amplifier. (See fig. 8-8.) When you connect the meter between the plate and cathode of the electron tube, the meter resistance is in parallel with the effective plate resistance Thus, the plate resistance is lowered. The effective plate resistance is in series with the plate load resistor and this series circuit appears across the supply voltage as a voltage divider. Since the overall 8-6

7 high input impedance. The TS-505 multimeter contains a VTVM, and it is used extensively in electronics maintenance. Figure 8-8. Loading effect created by meter resistance. You should refer to figure 8-9 as you read this section. The VTVM measures dc voltages from 0.05 volt to 1,000 volts (in nine ranges) and ac voltages from 0.05 volt to 250 volts rms (in seven ranges) at frequencies from 30 Hz to 1 MHz. Using the RF adapter with the dc voltage measurement circuit lets you measure RF voltages from 0.05 volt to 40 volts rms at frequencies from 500 khz to 500 MHz. You may measure resistances from 1 ohm to 1,000 megohms. resistance is now lower, the current through R L will increase. This causes the voltage drop across R L to also increase, and the voltage drop across R eff will decrease. The result is an incorrect indication of plate voltage and is called the loading effect. The lower the sensitivity of the meter, the greater the loading effect and the higher the incorrect indication (error) will be. A meter having a sensitivity of 20,000 ohms per volt and a 250-volt maximum scale reading would introduce an error of about 1 percent. However, in circuits with very high impedances, even a meter with a 20,000-ohm-per-volt sensitivity would impose too much of a load on the circuit. VACUUM TUBE VOLTMETER Another limitation of the ac, rectifier-type voltmeter is the shunting effect at high frequencies of the relatively large capacitance of the meter s rectifier. This shunting effect may be greatly reduced by replacing the usual metallic oxide rectifier with a diode electron tube. The output of the diode goes to the grid of an amplifier, in which the plate circuit contains the dc meter. Such a device is an electron tube voltmeter or a vacuum tube voltmeter (VTVM). Voltage measurements are extremely accurate with this type of meter, even at frequencies up to 500 megahertz and sometimes higher. The VTVM model that is used determines its frequency limitation. The input impedance of a VTVM is large; therefore, the current drawn from the circuit voltage being measured is small and in most cases negligible. The main purpose of a VTVM is to reduce the loading effect by taking advantage of the VTVM s extremely Figure 8-9. TS-505 multimeter front panel. 8-7

8 The accuracy of this meter is ±5 percent for dc voltages and ±6 percent for ac and RF voltages. The meter movement requires 1 ma for full-scale deflection. The input impedance to the meter is 6 megohms at audio frequencies, 40 megohms on the 1,000-volt dc range, and 20 megohms on all other ranges. The power requirement is 98 to 132 volts, single phase, 50 to 1,000 Hz, at about 21 volt-amperes. The removable cover of the TS-505 contains accessories such as alligator clips, an RF adapter, and miniature probe tips. The miniature tips slip over the regular tips for work in confined areas. Operating Controls The following are the controls you use when operating the meter (fig. 8-9): FUNCTION switch Selects the type of multimeter operation desired and turns the multimeter on or off. RANGE switch Selects the various voltage or resistance measurement ranges. ZERO ADJ. control Controls the pointer of the indicating meter. Use it to set the meter pointer at zero on the +DC, DC, AC, or OHM scale, or at midscale on the ±DC scale. OHMS ADJ. control Controls the pointer of the indicating meter. Use it to set the meter pointer at on the OHMS scale when the FUNCTION switch is set on OHMS position. Meter Indicates the value of voltage or resistance measured. AC LINE cord Connects the multimeter to the ac power source. COMMON probe Connects the ground or common circuit of the multimeter to the equipment under test. DC probe Connects the equipment under test to the dc measuring circuit of the multimeter OHMS probe Connects the equipment under test to the ohmmeter circuit of the multimeter AC probe Connects the equipment under test to the ac measuring circuits of the multimeter. Pilot light indicator Lights when power is applied to the multi meter. Techniques for Use The TS-505 multimeter is not difficult to operate. However, do not try to use this instrument unless you have studied the technical manual that contains the operating procedures, or unless you have received instruction in its proper use from your shop supervisor. There are two peculiarities of this meter that you need to know about. 1. It must warm up before it gives accurate readings. This usually takes about 10 minutes. During this period, the meter pointer may drift rapidly. This is normal. 2. You cannot read voltage measurements directly off the meter scale when the function switch is in the ±DC position. The purpose of the ±DC position (zero center scale) is to determine the polarity of an unknown dc voltage. It also indicates a zero dc voltage input to the multimeter CAUTION The maximum input dc voltage to the multimeter when in the ±DC position is one-half of the range switch voltage setting. The major difference between any VTVM and a conventional multimeter is that the VTVM uses a vacuum tube in its input. For a detailed explanation of the circuitry of the TS-505 VTVM, consult the manufacturer s manual or the operation and service instruction manual. PHASE ANGLE VOLTMETER The overall accuracy of many electronic equipments is determined by measuring phase angles. In the past, the phase shift or phase angles between signals were measured by observing patterns on an oscilloscope. It was hard to determine small angles and difficult to translate various points into angles and sines of angles using this method. Also, using oscilloscope patterns is 8-8

9 a limiting factor if one of the signals contains harmonic distortion or noise. In any complex waveform containing a fundamental frequency and harmonics, measuring phase shifts presents problems. In most applications, the primary interest is the phase relationship of the fundamental frequency, regardless of the phase relationship of any harmonics that are present. Therefore, one requirement of a phasemeasuring device is its ability to measure the phase difference between two discrete frequencies, regardless of the phase and amplitude of other components of the waveform. Figure 8-10 shows the basic block diagram of a phase angle voltmeter. There are two inputs the signal and the reference. Each channel contains a filter that passes only the fundamental frequency and highly attenuates all other frequencies. Each channel has a variable amplitude control and amplifiers to increase the variety of signals that you can check. A calibrated phase shifter is inserted into one channel. That channel signal can then be phase shifted to correspond to the other channel. The phase detector detects this and indicates it on the meter. The calibrated phase shifter is a switch (whose position corresponds to the 0-degree, 90-degree, 180-degree, and 270-degree phase shift) and a potentiometer (whose dial is calibrated from 0 to 90 degrees). The total phase shift is the sum of the two readings. The phase detector is a balanced diode, bridgetype demodular. Its output is proportional to the signal frequency amplitude times the cosine of the angle of phase difference between the signal input and the reference input. If the shifted reference input is in phase or 180 degrees out of phase with the signal input, the output from the phase detector is proportional to the signal input amplitude. The cosine of the angle is unity. If the shifted reference input is 90 degrees or 270 degrees from the signal input, the phase detector output will be zero (the cosine of the angle is zero). The point at which the two signals are in phase or 180 degrees out of phase is the point of Figure Phase angle voltmeter block diagram. 8-9

10 maximum deflection on the meter. The difference Q8. between the in-phase and the 180-degree out-ofphase points is in the direction in which the needle swings not the distance it swings. Upon approaching the point of maximum deflection, the rate of change of the meter reading decreases because the cosine has a small rate of change near 0 degrees. This makes it difficult to read the exact point of maximum deflection. Q9. Loading effect is the result of a meter s sensitivity, and it causes incorrect voltage indications. What relationship exists between a meter s sensitivity and its loading effect? What is the major difference between a VTVM and a conventional multimeter? The cosine s maximum rate of change occurs as it approaches 90 degrees (and thus gives a better indication on the meter). Therefore, most commercial voltmeters are set to determine the point at which the signals are 90 degrees out of phase, known as quadrature. However, this requires converting the phase shifter reading so it shows the correct amount of phase shift rather than 90 degrees more or less than the actual amount. Different manufacturers use different methods to determine the signal quadrant, which leads to some confusion. Also, manufacturers differ on whether the final reading is a leading or a lagging phase shift. This means that you, the technician, must know the phase angle voltmeter you are using. The Navy has several phase angle voltmeters and each operates differently. You cannot assume that the method you use to determine the phase angle on one type of meter is the method you should use to determine it on another. Also, you cannot assume that because one meter gives a leading angle between signal and reference waveforms, another meter will also give a leading phase shift. Q5. Q6. Q7. What is the most accurate portion of the ohmmeter scale, and why? When repairing equipment, you should use a meter with the same sensitivity as specified in schematics and wiring diagrams. What is the reason for doing this? Name the piece of test equipment that consists of a hand-driven dc generator, applies a high voltage to the component under test, and measures current leakage. Q10. A phase angle voltmeter is used to determine the overall accuracy of electronic equipment by measuring phase angles. What is actually measured by the phase angle voltmeter? DIFFERENTIAL VOLTMETER The differential voltmeter is a reliable precision piece of test equipment. Its general function is to compare an unknown voltage with an internal reference voltage and to indicate the difference in their values. A common differential voltmeter is the 883A (fig. 8-11), manufactured by the John Fluke Co. The Fluke 883A has many capabilities and uses. You may use it as a conventional transistor voltmeter for measuring voltages from 0 volt to 1,100 volts dc, a differential voltmeter for precision (0.01 percent of input voltage) measurement of dc voltages in this range, or as an accurate ac voltmeter and a megohmmeter for measuring resistance from 10 megohms to 11,000 megohms. The Fluke Model 883A is accurate enough for precision work in calibration laboratories yet rugged enough for general shop use. For more information on the Fluke Model 883A, you should refer to NEETS, module 16. FREQUENCY MEASUREMENT Often, frequency measurements are an essential part of preventive and corrective maintenance for electronic equipment. You may have to determine rotation frequencies of some mechanical devices. For example, you have to 8-10

11 Figure Fluke Model 883A differential voltmeter. check the output frequency of electric power generators when starting the engine and during preventive maintenance routines. Equipment that operates in the audio-frequency range requires adjusting to operate at the correct frequencies. Accurate tuning of radio transmitters to their assigned frequencies provides reliable communications. Tuning also avoids interfering with radio circuits operating on other frequencies. Radar sets also require proper tuning to get satisfactory performance. A stroboscope can measure the rotation frequency of rotating machinery such as radar antennas, servomotors, and other types of electric motors. Stroboscopic methods compare the rate of one mechanical rotation or vibration with another or with the frequency of a varying source of illumination. Tachometers can also measure the rotation frequency of armatures in electric motors, dynamotors, and engine-driven generators. Vibrating-reed, tuned-circuit, or moving-disk meters directly measure the electrical output frequency of ac power generators. The vibratingreed device is the simplest frequency meter, and it is rugged enough to mount directly on generator control panels. You may also use it to check the line voltage in the shop to be sure the proper 8-11

12 frequency is available to the equipment and/or test sets. Frequency Meters The term frequency meter refers to an item of test equipment used to indicate the frequency of an external signal. Although some frequency meters generate signals having a basic frequency, you should not confuse them with test equipment known as signal generators. The frequency meter measures the frequency of a signal developed in an external circuit. Some frequency meters generate a signal frequency; others do not. Those that don t generate an internal frequency are known as wavemeters. There are two basic types of wavemeters reaction and absorption. Frequency meters that do generate an internal frequency may use either electronic or mechanical oscillation as the frequency generator. Measurement Methods You in the parison may make frequency audio-frequency range method or by using a measurements by the comdirect-reading frequency meter. You may make frequency comparisons by use of a calibrated audiofrequency signal generator with either an oscilloscope or a modulator and a zero-beat indicator device. Instruments using series frequency-selective electrical networks, bridge test sets having null indicators, or counting-type frequency meters can make direct-reading frequency measurements. Since the wavemeter is relatively insensitive, it is very useful in determining the fundamental frequency in a circuit generating multiple harmonics. You may check the calibration of test equipment that measures signals in this frequency range by comparing them with standard frequency signals broadcast by the National Bureau of Standards. The signal frequencies of radar equipment that operate in the UHF and SHF ranges can be measured by resonant cavity-type wavemeters, resonant coaxial line-type wavemeters, or Lecher-wire devices. When properly calibrated, resonant cavity and resonant coaxial line wavemeters are more accurate. They also have better stability than wavemeters used for measurements in the LF to VHF range. These frequency-measuring instruments often come as part of communication and electronic equipment, but they are also available as general-purpose test sets. 8-12

13 Heterodyne Meters A mixer or detector Heterodyne frequency meters are available in several varieties. Although they all function in the same general manner, some differences exist in how they accomplish their purpose. Test instruments of this class generate a signal within the test set. This signal mixes with a signal from the equipment under test to obtain a beat frequency. The frequency of one signal is then changed to obtain a zero beat. The beat frequency is the difference frequency that results from heterodyning two signals. A zero beat results when heterodyning two signals of the same frequency. You may determine the frequency of the unit under test by reading the frequency indicator of the test set. A heterodyne frequency meter (fig. 8-12) usually consists of the following parts: A heterodyne oscillator An RF harmonic amplifier A crystal-controlled oscillator A modulator An AF output amplifier A means for indicating frequency Most models come with a set of calibration charts giving the dial readings for the frequencies listed and a table of the crystal harmonics. The table and charts give complete and accurate frequency coverage over the set s range. Some models indicate the frequency directly on dials. The crystal-controlled oscillator operates at a fixed frequency. However, it is also capable of emitting various harmonic frequencies of the crystal for use as check frequencies. These checkpoints provide a measure for adjusting the heterodyne oscillator, thus ensuring more accurate operation. Provisions are usually made within the crystal-controlled oscillator for precise adjustment to its assigned fundamental frequency. Figure Crystal-calibrated heterodyne frequency meter block diagram. 8-13

14 Wavemeters Wavemeters are calibrated, resonant circuits used to measure frequency. Although not as accurate as heterodyne frequency meters, wavemeters are comparatively simple and easy to carry. You may see any type of resonant circuit in wavemeter applications. The exact kind of circuit depends on the frequency range for which the meter is intended. Resonant circuits consisting of coils and capacitors are used with low-frequency wavemeters. VHF and microwave instruments have butterfly circuits, adjustable transmission line sections, and resonant cavities. There are three basic kinds of wavemeters the absorption, the reaction, and the transmission types. The absorption wavemeter consists of the basic resonant circuit, a rectifier, and a meter for indicating the amount of current induced into the wavemeter. In use, this type of wavemeter loosely couples to the measured circuit. Then, you adjust the resonant circuit of the wavemeter until the current meter shows a maximum deflection. You determine the frequency of the circuit under test from the calibrated dial of the wavemeter. The reaction wavemeter gets its name from having to be adjusted until a marked reaction occurs in the circuit being measured. For example, the wavemeter is loosely coupled to the grid circuit of an oscillator, and the tuning circuit of the wavemeter is adjusted until it is in resonance with the oscillator frequency. The setting of the wavemeter dial is made by observing the gridcurrent meter in the oscillator. At resonance, the wavemeter circuit takes energy from the oscillator, causing the grid current to dip sharply. The frequency of the oscillator is then determined from the calibrated dial of the wavemeter. This type is commonly referred to as a grid-dip meter. The transmission wavemeter is an adjustable coupling link. When inserted between a source of radio-frequency energy and an indicator, energy is transmitted. However, energy to the indicator only occurs when the wavemeter is tuned to the frequency of the source. Transmission wavemeters are commonly used to measure microwave frequencies. Units of this type are also found in echo boxes. The additional provisions for echo boxes permit additional testing functions. Many types of wavemeters are used for various functions. The cavity-type wavemeter (fig. 8-13) is the type most commonly used for measuring microwave frequencies; therefore, it is the one covered in this chapter. The device employs a resonant cavity that effectively acts as Figure Typical cavity wavemeter. 8-14

15 a high-q, LC tank circuit. The resonant frequency of the cavity varies by means of a plunger, which mechanically connects to a micrometer mechanism. Movement of the plunger into the cavity reduces the cavity size and increases the resonant frequency. Conversely, an increase in the size of the cavity (made by withdrawing the plunger) lowers the resonant frequency. The microwave energy from the equipment under test goes into the wavemeter through one of two inputs A or D. The crystal rectifier then detects (rectifies) the signal, and the current meter (M) indicates the rectified current. You can use the cavity wavemeter as either a transmission-type or an absorption-type wavemeter. When used as a transmission wavemeter, the unknown signal couples into the circuit through the A input. When the cavity is tuned to the resonant frequency of the signal, energy is coupled through coupling loop B into the cavity and out through loop C to the crystal rectifier. It is rectified, and current flow resulting from this rectification is indicated on the meter. At frequencies off resonance, little or no current flows in the detector, and the meter reading is small. Vary the micrometer and attached plunger until you get a maximum meter reading. Compare the resulting micrometer setting with a calibration chart supplied with the wavemeter to determine the unknown frequency. When the unknown signal is relatively weak, such as the signal from a klystron oscillator, the wavemeter functions as an absorption wavemeter. Connect the instrument at the D input. The RF loop C then acts as an injection loop to the cavity. When the cavity is tuned to the resonant frequency of the klystron, the cavity absorbs maximum energy and the meter will dip. This indicates a reduction of current. When the cavity is not at the resonant frequency of the klystron, the current meter will indicate high current. Therefore, tune the cavity for a minimum reading, or dip, in the meter, and determine the resonant frequency from the micrometer setting and the calibration chart. Potentiometer R1 adjusts the sensitivity of the meter from the front panel of the instrument. J1 is a video jack for observing video waveforms with a test oscilloscope. A directional antenna is used with the instrument for making relative field strength measurements of radiated signals for use in measuring the frequency of radar transmitters. This setup is also used for constructing radiation patterns of transmitting antennas. In radiation pattern measurements connect the directional antenna to the wavemeter input and tune the instrument to the frequency of the system under test. The cavity will then lock on this frequency by an automatic frequency control (AFC) system. For reliable results, the output signal must be continuous and constant. This is necessary for any variation in the meter reading caused directly by a change in the actual field strength. That is the signal field strength when the position of the wavemeter changes with respect to the transmitting antenna. After establishing a reference level on the meter, change the position of the wavemeter by moving it around the radiating antenna, maintaining a fixed distance from it. To determine the field pattern, record the wavemeter readings at various positions around the transmitting equipment on polar graph paper. COUNTER-TYPE FREQUENCY METER The counter type of frequency meter is a high-speed electronic counter, with an accurate, crystal-controlled time base. This type of combination provides a frequency meter that automatically counts and displays the number of events (hertz) occurring in a precise interval, The frequency meter itself does not generate any signal, it merely counts the recurring pulses fed to it. The Hewlett-Packard Model 5245L electronic counter (figs and 8-15) is a high-frequency general-purpose electronic counter. The Model 5245L measures frequencies from 0 to 50 MHz, periods from 1 µsec to 10 seconds, and period averages from 10 to 100,000 periods. Also, it can measure the ratio of two frequencies and the multiplied ratio of two frequencies. The Model 5245L provides the following additional features: Decade scaling to to 50 MHz for any frequency Standard output frequencies from 0.1 Hz to 10 MHz, in decade steps Four-line, binary-coded-decimal (BCD) output to drive digital recorder (Hewlett- Packard Model 562A), digital-to-analog converter (Hewlett-Packard Model 580A/581A), remote readout, or data processing equipment Remote control by external contact closure 8-15

16 Figure Model 5245L electronic counter front panel. 8-16

17 Figure Model 5245L electronic counter rear panel. 8-17

18 Display storage that permits reading display while making a new count Eight-digit display using rectangular (narrow) digital display tubes, with decimal point position and measurement units displayed automatically Operation with plug-in units that extend the basic range and performance of the counter The Model 5245L features solid-state design, low-power consumption, small size (5 1/4-inch panel height), light weight (32 pounds), easy conversion for rack mounting, and modular plugin circuit boards for simplified maintenance. To increase the range of measurement, five plug-in units (not shown) are available. The Model 5245L measures frequency, period average, ratio of two frequencies, and total events. A FUNCTION selector switch selects measurement function, and a TIME BASE selector switch selects time base or multiplier. A SAMPLE RATE control selects the sampling rate, and a SENSITIVITY control adjusts instrument sensitivity. Direct readout is available in both PERIOD and FREQUENCY functions with measurement units displayed and with decimal point automatically positioned. In the MANUAL function the display is a direct read. The decimal point will not light. Note that the only difference between ratio and period measurements is the use of an external frequency instead of the internal 1-MHz oscillator. Two factors determine the basic counter accuracy, One factor is the aging rate of the 1-MHz crystal standard in the time base, which is less than 2 parts in per week. A second factor is the inherent error of ±1 count present in all counters of this type. This error is due to phasing between the timing pulse that operates the electronic gate and the pulses that pass through the gate to the counters. The chart in figure 8-16 shows the errors possible for frequency or period measurements, The three factors contributing to the accuracy of period measurements are as follows: The aging rate of the l-mhz standard, which is less than 2 parts in per week The ambiguity of the ±1 count The ± trigger error (for one period, and a signal-to-noise ratio of 40 db, this trigger error is 0.3 percent at rated sensitivity) Figure Model 5245L electronic counter measurement accuracy. Frequencies of 0.1 Hz to 1 MHz are available in decade steps at the TIME BASE EXT connector as selected by the TIME BASE switch. This output is subject to the following restrictions, Frequencies of 0.1 Hz through 10 MHz are available in decade steps at the rear-panel OUTPUT connector as selected by the rear-panel OUTPUT switch. This output is subject to the following restrictions. All frequencies are available one at a time in the MANUAL function without interruption. 1 khz is continuously available for all functions except 100K PERIOD AVERAGE. The 10 khz to 10 MHz is continuously available in all functions. NOTE: The accuracy and stability of these outputs are the same as those of the time base oscillator. The Hewlett-Packard Model 525 1A frequency converter extends the frequency range of the Model 5245L to 100 MHz. The Model 5251A mixes a selected 10-MHz harmonic (between 20 and 90 MHz) with the input signal. The resulting difference-frequency signal receives amplification and goes to the basic counter for counting and display. Because the selected 10-MHz harmonic 8-18

19 is from a harmonic generator driven by a 10-MHz output from the basic counter, the stability and accuracy of the basic counter remains. The Hewlett-Packard Model 5253B frequency converter extends the frequency range of the Model 5245L to 512 MHz. To retain the stability and basic accuracy, multiply a 10-MHz signal, from the counter s internal time base, to a known harmonic frequency. When this harmonic frequency mixes with the input signal frequency, the difference frequency that results is within the range of the basic counter, and the counter displays the difference frequency. The Hewlett-Packard Model 5254A frequency converter provides the Model 5254L with a frequency range from 300 to 3,000 MHz. To retain the stability and accuracy of the basic counter, use a 50-MHz multiple of the crystaloscillator signal from the counter to beat with the measured signal. The difference frequency produced is within the display range of the basic counter. The converter has an indicator that aids in frequency selection and indicates the output level to the counter. The required input signal level is 50 mv rms to 1 V rms. The input connector is a type N female. The Hewlett-Packard Model 5261A video amplifier unit extends the sensitivity of the Model 5245L to 1.0 millivolt over the frequency range of 10 Hz to 50 MHz. Input impedance increases to 1 megohm and can increase to 10 megohms by using an accessory 10:1 divider probe (Hewlett- Packard 10003A) for signals greater than 10 mv. A 50-ohm output is used for oscilloscope monitoring of the amplified signal. The Hewlett-Packard Model 5262A time interval unit provides start and stop pulses. These pulses start by electrical inputs to the main count gate in the Model 5245L, enabling it to make time measurements. Time intervals from 1 microsecond to 10 8 seconds are measured with a resolution of 0.1 microsecond. Basic counter accuracy remains when the signal counted is from the internal oscillator. Q11. Q12. Q13. Describe the general function of a differential voltmeter. What item of test equipment is used to indicate the frequency of an external signal? List the parts of most heterodyne frequency meters. Q14. Q15. Q16. Q17. Wavemeters are calibrated resonant circuits used to measure frequency. List the three basic kinds of wavemeters. Of the three basic wavemeters, which one is commonly used to measure microwave frequencies? The counter frequency meter is a high-speed electronic counter, with an accurate, crystalcontrolled time base. What does this combination provide? What does the Model 5245L counter frequency meter measure? POWER MEASUREMENTS You must check the power consumption and the input and output signal power levels of electronic equipment. It is easy to determine dc power; the unit of power (the watt, P) is the product of the potential in volts (E) and the current (I) in amperes, or, P = IE. You can take a few basic circuit measurements and compute the power using Ohm s law. It is not as easy to determine ac power. To make ac power measurements, you must consider the phase angle of the voltage and current. Measurement is further complicated by the frequency limitations of various power meters. If there is no phase difference, compute ac power in the same manner as dc power by determining the average value of the product of the voltage and current. Electric power at a line frequency of approximately 60 Hz is directly measured by a dynamometer type of wattmeter. This type of meter indicates the actual power. Therefore, the phase angle of the voltage and current does not have to be determined. Normally, the exact power consumption of equipment is not necessary for maintenance, and a current measurement is enough to decide whether the power consumption is within reasonable limits. Many ac voltmeters have scales calibrated in decibels (db) or volume units. Such meters are used to make measurements where direct indication in decibels is desired. Remember, these are voltmeters and that power measurements are not meaningful unless the circuit impedance is known. The topic of decibels is discussed in chapter 1 of Aviation Electronics Technician 3, NAVEDTRA 14028, NEETS, modules 11 and 16, and in the Electronics Installation & Maintenance 8-19

20 Book Test Methods and Practices, NAVSHIPS 0967-LP For more information on decibels, refer to these publications. At radio frequencies below the UHF range, power is usually determined by voltage, current, and impedance measurements. One common method used to determine the output power of RF oscillators and radio transmitters consists of connecting a known resistance to the equipment output terminals. After measuring the current flow through the resistance, you then calculate the power as the product of I 2 R. Since the power is proportional to the current squared, the meter scale can indicate power units directly. A thermocouple ammeter is used to measure RF current. The resistor used to replace the normal load is of special design. It has to have low reactance and the ability to dissipate the required amount of power. Some common names for such resistors are dummy loads or dummy antennas. In the UHF and SHF portions of the RF spectrum, it is more difficult to accurately measure voltage, current, and impedance. These basic measurements may change greatly at slightly different points in a circuit. Also, small changes in the placement of parts near the tuned circuits may affect their measurements. Test instruments that convert RF power to another form of energy, such as light or heat, can measure the power output of microwave radio or radar transmitters indirectly. One method measures the heating effect of a resistor load on a stream of passing air. To achieve accurate measurement of large magnitude power, you can measure the temperature change of a water load. The most common type of power meter for use in this frequency range uses a bolometer. The bolometer is a loading device that undergoes changes of resistance as changes in the power dissipation occur. Measure the resistance before and after applying RF power; the change in resistance determines the power. The Model 432A power meter operates with Hewlett Packard (HP) temperature-compensated thermistor mounts, such as the 8478B and 478A coaxial and 486A waveguide series. The frequency range of the 432A with these mounts in 50-ohm coaxial systems is 10 MHz to 18 GHz. Its frequency range in waveguide systems is 2.6 GHz to 40 GHz. Full-scale power ranges are 10 microwatts to 10 milliwatts (-20 dbm to +10 dbm). The total measurement capacity of the instrument is divided into seven ranges, selected by a front-panel RANGE switch (fig. 8-17). The COARSE ZERO and FINE ZERO controls zero the meter. Zero carry-over from the most sensitive range to the other six ranges is within ±0.5 percent. When setting the RANGE A11. Its general function is to compare an unknown voltage with an internal reference voltage and to indicate the difference in their values. A12. Frequency meter. A13. A heterodyne oscillator, RF harmonic amplifier, crystal-controlled oscillator, a mixer or detector, a modulator, an AF output amplifier, and a means for indicating frequency. A14. Absorption, reaction, and transmission. A15. Transmission. A16. A frequency meter that automatically counts and displays the number of events (hertz) occurring in a precise interval. A17. Frequency, period average, ratio of two frequencies, and total events. 8-20

21 Figure Model 432A power meter front panel. switch to COARSE ZERO, the meter indicates thermistor bridge unbalance. Adjust the front panel COARSE ZERO adjust for initial bridge balance. For best results, FINE ZERO the 432A on the particular meter range in use. The CALIBRATION FACTOR switch provides discrete amounts of compensation for measurement uncertainties related to standing wave ratio (SWR) and thermistor mount efficiency. The calibration factor value permits direct meter reading of the RF power delivered to an impedance equal to the characteristic impedance (Z O ) of the transmission line between the thermistor mount and the RF source. The label of each 8478B, 478A or 486A thermistor mount contains calibration factor values. The MOUNT RESISTANCE switch on the front panel compensates for three types of thermistor mounts. You can use Model 486A waveguide mounts by setting the MOUNT RESISTANCE switch to 100 or 200Ω, depending on the thermistor mount. The 200Ω position is for use with Models 478A and 8478B thermistor mounts. The rear panel baby N connector (BNC) labeled RECORDER (fig. 8-18) provides an output voltage that is Figure Model 432A power meter rear panel. linearly proportional to the meter current. One volt fed into an open circuit equals full-scale meter deflection. This voltage develops across a 1-kilohm resistor. Therefore, when a recorder with a 1-kilohm input impedance is connected to the RECORDER output, about 0.5 volt will equal full-scale deflection. This loading of the RECORDER output has no effect on the accuracy of the 432A panel meter. You may connect a digital voltmeter to the rear panel RECORDER output for more resolution of power meter readings. When connecting a voltmeter with an input impedance greater than 1 megohm to the RECORDER output, 1 volt equals full-scale deflection. The 432A has two calibration jacks (V RF and V COMP ) on the rear panel. You can use them for precision power measurements. Instrument error can be reduced from ±1 percent to ±0.2 percent of reading +5 µw. This depends on the care taken when measuring and on the accuracy of auxiliary equipment. Some factors affect the overall accuracy of power measurement. The major sources of error are mismatch error, RF losses, and instrumentation error. In a practical measurement situation, both the source and thermistor mount have SWR, and the 8-21

22 source seldom matches the thermistor mount unless using a tuner. The amount of mismatch loss in any measurement depends on the total SWR present. The actual thermistor mount impedance, the electrical length of the line, and the characteristic impedance of the line will determine the impedance that the source sees. In general, neither the source nor the thermistor mount has impedance, and the actual impedances are only reflection coefficients, mismatch losses, or SWR. The power delivered to the thermistor mount, hence the mismatch loss, can only be described as being somewhere between two limits. The uncertainty of power measurement due to mismatch loss increases with SWR. Limits of mismatch loss are generally determined by means of a chart. To determine the total mismatch loss uncertainty in power measurement, algebraically add the thermistor mount losses to the uncertainty caused by source and thermistor mount match. RF losses account for the power entering the thermistor mount but not being dissipated in the detection thermistor element. Such losses may be in the walls of a waveguide mount or in the center conductor of a coaxial mount. Losses may also be from the capacitor dielectric, poor connections within the mount, or be due to radiation. The degree of inability of the instrument to measure the substitution power supplied to the thermistor mount is called power meter accuracy or instrumentation error. Instrumentation error of the Model 432A is ±1 percent of full scale, 0 C to +55 C. Calibration factor and effective efficiency are correction factors for improving power measurement accuracy. Both factors are marked on every HP thermistor mount. The calibration factor compensates for thermistor mount VSWR and RF losses whenever connecting the thermistor mount to an RF source without a tuner. Effective efficiency compensates for thermistor mount RF losses when using a tuner in the measurement system. Set the 432A CALIBRATION FACTOR selector to the appropriate factor indication on the thermistor mount. This resulting power indicates the power that would go from the source to a load impedance equal to The calibration factor does not compensate for source VSWR or for multiple reflections between the source and the thermistor mount. You can minimize mismatch between the source and the thermistor mount without a tuner. Insert a low SWR precision attenuator in the transmission line between the thermistor mount and the source. Since the mount impedance (and corresponding SWR) deviates significantly only at the high and low ends of a microwave band, it is unnecessary to use a tuner. A tuner or other effective means of reducing mismatch error is recommended when the source SWR is high or when more accuracy is necessary. The HP Model 478A coaxial thermistor mount (fig. 8-19) is designed for use with HP Models 431 and 432 power meters. It can measure microwave power from 1 µw to 10 µw. The mount design minimizes adverse effects from environmental temperature changes during measurement. For increased measurement accuracy, effective efficiency and calibration factor are measured for each mount and at selected frequencies across the operating range. The results are marked on the label of the instrument. The Model 478A operates over the 10-MHz to 10-GHz frequency range. Throughout the range, the mount terminates the coaxial input in a 50-ohm impedance and has a SWR of not more than 1.75 without external tuning. Each mount contains two matched series pairs of thermistors, which cancel the effects of drift with ambient temperature change. Thermal stability is accomplished by mounting the leads of all four thermistors on a common thermal conductor to ensure a common thermal environment. This conductor is thermally insulated from the main body of the mount. The thermal insulation makes sure thermal noise or shocks applied externally to the mount, such as those Figure Model 478A thermistor mount. 8-22

23 from handling the mount manually, cannot significantly disturb the thermistor. The thermal immunity lets the thermistors be used to measure microwave power down to the microwatt region. Q18. By what method is dc power determined? Q19. You use a resistor that is specially designed to dissipate the required amount of power and replace normal loads. List the two types of resistors used for this purpose. Q20. List the major sources of error that affect the overall accuracy of power measurements. SEMICONDUCTOR TESTERS Since semiconductors have replaced vacuum tubes, the testing of semiconductors is vital. In this section, three basic types of equipment are discussed the Huntron Tracker 1000, Huntron Tracker 2000, and the Automatic Transistor Analyzer Model 900 in-circuit transistor tester. Huntron Tracker 1000 You will test components with Huntron Tracker 1000 using a two-terminal system, where two test leads attach to the leads of the component under test. The 1000 tests components in-circuit, even when there are several components in parallel. The following types of devices are tested using the Huntron Tracker 1000: Semiconductor diodes Bipolar and field effect transistors Bipolar and MOS integrated circuits (both analog and digital) Resistors, capacitors, and inductors The 1000 is used on boards and systems with ALL voltage sources in a power-off condition. A 0.25 ampere signal fuse (F1) connects in series with the channel A and B test terminals. Accidentally contacting test leads to active voltage sources (for example, line voltage, powered-up boards or systems, charged high-voltage capacitors, etc.) may cause this fuse to open, making replacement necessary. When the signal fuse blows, the display shows open circuit signatures, even with the test leads shorted together. CAUTION The device to be tested must have all power turned off and have all high-voltage capacitors discharged before connecting the 1000 to the device. The line fuse (F2) should only open when there is an internal failure inside the instrument. Therefore, you should always locate and correct the problem before replacing F2. The front panel of the 1000 makes function selection easy. The 1000 uses interlocking pushbutton switches for range selection. A toggle switch is used for channel selection, and integral LED indicators show the active functions. The CRT displays the signatures of the parts under test. The display has a graticule consisting of a horizontal axis that represents voltage, and a vertical axis that represents current. The horizontal axis is divided into eight divisions, which lets you estimate the voltage at which signature changes occur. This is mainly useful in determining semiconductor junction voltages under either forward or reverse bias. Push in the power on/off switch. The 1000 should come on line with the power LED illuminated. Before you can analyze signatures on the CRT, you must focus the To do this, turn the intensity control to a comfortable level. Now, adjust the focus control (back panel) for the narrowest possible trace. Aligning the trace is important in determining the voltages at which changes in the signature occur. With a short circuit on channel A, adjust the horizontal control until the vertical trace is even with the vertical axis. Open channel A and adjust the vertical control until the horizontal trace is even with the horizontal axis. Once set, you should not have to adjust these controls during normal operation. Turn the power off by pushing the power switch in. When you turn the power on again, the same intensity setting will be present. The 1000 has three impedance ranges low, medium, and high. To select these ranges, press the appropriate button on the front panel. Always start with the medium range; then you can adjust for other ranges. If the signature on the CRT is close to an open (horizontal trace), try the next higher range for a more descriptive signature. If the signature is close to a short (vertical trace), try the next lower range. There are two channels (channel A and channel B) that you can select by moving the 8-23

24 toggle switch to the desired position. When using a single channel, plug the red probe into the corresponding channel test terminal. Then plug the black probe into the common test terminal. When testing, connect the red probe to the positive terminal of the device (that is, anode, +V, etc.). Connect the black probe to the negative terminal of the device (that is, cathode, ground, and so forth.). By following this procedure, the signature will appear in the correct position on the CRT display. The alternate mode of the 1000 provide-s automatic switching back and forth between channel A and channel B. This allows easy comparison between two devices or the same point on two circuit boards. You select the alternate mode by moving the toggle switch to the ALT position. The alternate mode is useful when comparing a known good device with the same device whose quality is unknown. The signal section applies the test signal across two terminals of the device under test. The test signal causes current to flow through the device and a voltage drop across its terminals. The current flow causes a vertical deflection of the signature on the CRT display. The voltage across the device causes a horizontal deflection of the signature on the CRT display. The combined effect produces the current-voltage signature of the device on the CRT display. An open circuit has zero current flowing through the terminals and a maximum voltage across the terminals. In the LOW range, a diagonal signature from the upper right to the lower left of the CRT (fig. 8-20, view A) represents an open circuit. In the HIGH and MEDIUM ranges, an open circuit shows as a horizontal trace from the left to the right (fig. 8-20, view B). When you short the terminals together, the maximum current flows through the terminals, and the voltage at the terminals is zero. A vertical trace from the top to the bottom of the CRT graticule in all ranges shows this short (fig. 8-20, view C). The CRT deflection drivers boost the low-level outputs from the signal section to the higher voltage levels needed by the deflection plates in the CRT. The HORIZONTAL and VERTICAL controls on the front panel adjust the position of the trace on the CRT display. Figure Circuit signatures: View A Low-range open circuit; view B medium- and high-range open circuit; and view C all ranges short circuit. 288X 8-24

25 You use three other CRT controls to adjust the brightness and clarity of the trace INTENSITY, FOCUS, and ASTIGMATISM. The front panel intensity control is the primary means of adjusting the visual characteristics of the trace. The focus control is on the back panel and is operator adjustable. The astigmatism trim pot is inside the 1000 on the main printed circuit board. The pot is factory adjusted to the correct setting. Huntron Tracker 2000 The Huntron Tracker 2000 (fig. 8-21) is a versatile troubleshooting tool having the following features: Multiple test signal frequencies (50/60 Hz, 400 Hz, 2000 Hz) Four impedance ranges (low, medium 1, medium 2, high) Automatic range scanning Range control: high lockout Dual channel capability for easy comparison Large CRT display with easy to operate controls GENERAL OPERATION. You will test components using the 2000 t wo-terminal system. It also has a three-terminal system when using the built-in pulse generator. When using this system, you place two test leads on the leads of the component under test. The 2000 tests components in-circuit, even when there are several parts in parallel. Use the 2000 only on boards and systems with all voltage sources in a power-off condition. A 0.25 ampere signal fuse connects in series with the channels A and B test terminals. Accidental contact of the test leads to active voltage sources, such as line voltage, powered-up boards or systems, and charged high-voltage capacitors may cause this fuse to open, making replacement necessary. When the signal fuse blows, the 2000 displays short circuit signatures even with the test leads open. Adjustable rate of channel and/or range scanning alteration CAUTION Dual polarity pulse generator for dynamic testing of three terminal devices LED indicators for all functions The device under test must have all power turned off and all high-voltage capacitors discharged before connecting the 2000 to the device. Figure Huntron Tracker X 8-25

26 Table 8-1.-Front Panel Controls and Connectors 288X 8-26

27 Figure Front panel. 288X The line fuse should only open when there is an internal failure inside the instrument. Always locate the problem and correct it before replacing this fuse. Front Panel. The front panel of the 2000 makes function selection easy. All push buttons are the momentary action type. Integral LED indicators show which functions are active. Look at figure 8-22 and table 8-1 for details about each item on the front panel. Back Panel. Secondary controls and connectors are located on the back panel (fig and table 8-2). Figure Back panel. 288X Table 8-2.-Back Panel Controls and Connectors 288X 8-27

28 CRT Display. The signature of the part under test is displayed on the CRT. The display has a graticule consisting of a horizontal axis that represents voltage, and a vertical axis that represents current. The axes divide the display into four quadrants. Each quadrant displays different portions of the signatures. Quadrant 1 displays positive voltage (+V) and positive current (+I). Quadrant 2 displays negative voltage (-V) and positive current (+I). Quadrant 3 displays negative voltage (-V) and negative current ( I). Quadrant 4 displays positive voltage (+V) and negative current ( I). The horizontal axis divides into eight divisions, which allows the operator to estimate the voltage at which changes in the signature occur. This is useful in determining semiconductor junction voltages under either forward or reverse bias. OPERATION OF PANEL FEATURES. The following section explains how to use the front and back panel features. Turn the power/intensity knob clockwise. The 2000 comes on with the LEDs for power, channel A, 50/60 Hz, low range, and pulse/dc illuminated. Focusing the 2000 display is an important part of analyzing the test signatures. First you adjust the intensity control to a comfortable level. Then, adjust the focus control (back panel) for the narrowest possible trace. Aligning the trace is important in determining which quadrants the portions of a signature are in. With a short circuit on channel A adjust the trace rotation control until the trace is parallel to the vertical axis. Adjust the horizontal control until the vertical trace is even with the vertical axis. Open channel A and adjust the vertical control until the horizontal trace is even with the horizontal axis. Once set, you should not have to readjust these settings during normal operation. Range Selection. The 2000 has four impedance ranges low, medium 1, medium 2, and high. You select these ranges by pressing the appropriate button on the front panel. Start with one of the medium ranges; that is, medium 1 or medium 2. If the signature on the CRT display is close to an open (horizontal trace), select the next higher range for a more descriptive signature, If the signature is close to a short (vertical trace), select the next lower range. The high lockout feature, when activated, prevents the instrument from entering the high range. This feature works in either the manual or auto mode. The auto feature scans through the four ranges three with the HIGH LOCKOUT activated at a speed set by the RATE control. This feature allows you to see the signature of a part in different ranges while freeing your hands to hold the test leads. Channel Selection. There are two channels on the 2000-channel A and channel B. You select a channel by pressing the appropriate front panel button. When using a single channel, plug the red probe into the corresponding channel test terminal. Plug the black probe into the common test terminal. When testing, connect the red probe to the positive terminal of the device; that is, anode, +V, etc. Connect the black probe to the negative terminal of the device; that is, cathode, ground, and so forth. Following this procedure should assure that the signature appears in the correct quadrants of the CRT display. The ALT mode is a useful feature of the It lets you compare a known good device with a device of unknown quality. In this test mode, you use common test leads to connect two equivalent points on the boards to the common test terminal. The ALT mode of the 2000 allows you to automatically switch back and forth between channel A and channel B so you can easily compare two devices. You may also compare the same points on two circuit boards. Select the ALT mode by pressing the ALT button on the front panel. You may vary the alternation frequency by using the RATE control. NOTE: The black probe plugs into the channel B test terminal. When using the alternate and auto features simultaneously, each channel is displayed before the range changes. Figure 8-24 shows the sequence of these changes. Frequency Selection. The 2000 has three test signal frequencies 50/60 Hz, 400 Hz and 2000 Hz. You can select these by pressing the appropriate button on the front panel. In most cases, you should start with the 50/60 Hz test 8-28

29 control setting that selects the duty cycle determines the end of a pulse. The WIDTH control has no effect when in the dc mode. Troubleshooting Tips You will use the Huntron Tracker 1000 and the Huntron Tracker 2000 to test various types of devices and circuits. Some troubleshooting tips are given in this section. signal. Figure Auto/alternate sequence. 288X Use the other two frequencies to view small amounts of capacitance or large amounts of inductance. Pulse Generator. The built-in pulse generator of the 2000 allows dynamic, in-circuit testing of certain devices in their active mode. In addition to using the red and black probes, you use the pulse generator. The output of the pulse generator connects to the control input of the device under test with one of the blue micro clips provided. The pulse generator has two outputs, G1 and G2, so you can test three terminal devices in the alternate mode. A variety of output waveforms is available using the pulse generator selector buttons. First select the pulse mode or the dc mode using the PULSE/DC button. In the pulse mode, the LED flashes at a slow rate. In dc mode, the LED is continuously on. Then select the polarity of output desired using the positive (+) and negative ( ) buttons. All three buttons function in a push-on/push-off mode, and only interact with each other to avoid the NOT ALLOWED state. After selecting the specific output type, set the exact output using the LEVEL and WIDTH controls. The LEVEL control varies the magnitude of output amplitude from zero to 5 volts (peak or dc). During pulse mode, the WIDTH control adjusts the duty cycle of the pulse output from a low duty cycle to 50 percent maximum (square wave). The start of a pulse is triggered by the appropriate zero crossing of the test signal. This results in the pulse frequency being equal to the selected test signal frequency. The WIDTH Perform most tests using the medium or low range. Use the high range only for testing at a high impedance point, or if higher test voltages are required (that is, to test the Zener region of a 40-volt device). Sometimes, component defects are more obvious in one range than another. If a suspect device appears normal for one range, try the other ranges. Use the low range when testing a single bipolar junction, such as a diode, a baseemitter junction, or a base-collector junction. It offers the best signature. Use a higher range to check for reverse bias leakage. When performing in-circuit testing, do a direct comparison to a known good circuit. The 1000 test leads are not insulated at the tips, Be sure to make good contact to the device(s) under test. (NOTE: This tip pertains to the 1000 only.) When you troubleshoot, try relating the failure mode of the circuit under test to the type of defect the 1000 shows. For example, expect a catastrophic printed circuit board failure to have a dramatic signature difference from that of a normal device of the same type. A marginally operating or intermittent board may have a failed part that shows only a small pattern difference from normal. If you cannot relate a system failure to a specific area of the printed circuit board, begin by examining the signatures at the connector pins. This method of troubleshooting shows all the inputs and outputs. It will often lead directly to the failing area of the board. 8-29

30 Devices made by different manufacturers, especially digital integrated circuits, are likely to produce slightly different signatures. This is normal and may not show a failed device. Remember, leakage current doubles with every 10-degree Celsius rise in temperature. Leakage current shows up as a rounded transition (where the signatures show the change from zero current flow to current flow) or by causing curvature at other points in the signatures. Leakage current causes curvatures due to its nonlinearity. Never begin the testing of an integrated circuit using the low range. If you initially use the low range, confusion can result from the inability of this range to display the various junctions. Always begin testing using the medium range. If the signature is a vertical line, switch to the low range. Here you can check for a short or low impedance (less than 500 ohms). Switch to the low range if the device is suspect and appears normal in the medium range. This will reveal a defective input protection diode not evident when using the medium range. NOTE: The 2000 test leads are conductive only at the tips. Be sure to make good contact with the device(s) under test. When testing analog devices or circuits, use the low range. Analog circuits contain many more single junctions. Defects in these junctions show more easily when using the low range. Also, the 54-ohm internal impedance in the low range makes it less likely that parts in parallel with the device under test will sufficiently load the tester to alter the signature. When testing an op amp in-circuit, compare it directly to a known good circuit. This is because the many different feedback paths associated with op amps can cause an almost infinite number of signatures. Often when checking a Zener diode in-circuit, it will not be possible to examine the Zener region due to circuit leakage. If you must see the Zener region under this condition, unsolder one side of the diode to eliminate the loading effects of the circuit. HUNTRON TRACKER Bipolar integrated circuits containing internal shorts produce a resistive signature (a straight line). This line begins in the 10 o clock to 11 o clock position. It ends in the 4 o clock to 5 o clock position on the display when using the low range. This type of signature is always characteristic of a shorted integrated circuit. It results from a resistive value of 4 to 10 ohms, typical of a shorted integrated circuit. A shorted diode, capacitor, or transistor junction always produces a vertical (12 o clock) straight line using the low range. HUNTRON TRACKER Bipolar integrated circuits containing internal shorts produce a resistive signature (a straight line) beginning in the 1 o clock to 2 o clock position. This signature ends in the 7 o clock to 8 o clock position when using the low range. This type of signature is characteristic of a shorted integrated circuit. This results from a resistive value of 4 to 10 ohms. A shorted diode, capacitor, transistor junction, etc., always produces a vertical (12 o clock) straight line when using the low range. Automatic Transistor Analyzer Model 900 You can use this instrument to test bipolar transistors and diodes in any one of three different modes. Two modes, the VIS and SND, can be used either in-circuit or out-of-circuit. In the VIS mode, red and green lights flashing in or out of phase with the amber light show the condition of the device under test. In the SND mode, the Sonalert also indicates good devices by beeping out of phase with the amber light. The intent of the SND mode is to permit the operator to perform in-circuit tests on transistors or diodes without having to look at the light display. The third mode is the METER mode. You can only use this for out-of-circuit testing. In the METER mode, you may measure Beta, and material identity. Also, you can measure emitter base voltage, base current (Ib), and collector current (Ic). There are four ranges for the Beta mode one for small signal transistors, two ranges for medium-power transistors, and one for large-power transistors. In the VIS mode and the SND mode, the maximum voltage, current and signal levels applied to the device under test are within safe limits. Therefore, the device under test will receive no damage nor will any adjacent circuitry. This instrument will test transistors and diodes in-circuit in the VIS or SND mode if the total dynamic shunt impedances across the junctions are not less than 270 ohms. Also, the total 8-30

31 dynamic shunt for the emitter to collector must not be less than 25 ohms. If such should occur, the test set will give the indication for a SHORT. The 8-inch meter, which reads from left to right, has two scales marked 0-10 and The 0-10 range is used in the leakage collector current and Vbe (IDENT) modes. The 0-50 range is used in the BETA modes. Notice a mark on the meter just short of half-scale with the nomenclature GERMANIUM and SILICON. This mark is the reference in the IDENT mode. As the meter markings show, those readings below the mark show the device material is germanium. The readings above the mark show the device is silicon. On the slanting horizontal panel immediately in front of the meter face are the appropriate test sockets and two push-button switches. One switch is ZERO and the other BETA. On the vertical front panel immediately below the push-button switches are knobs marked ADJ and CAL. At the top center is the POLARITY switch marked PNP and NPN. In the center of the vertical front panel is the RANGE switch, the FUNCTION switch, and the Sonalert. Near the bottom of the vertical front panel are the probe jacks. The slide switch for turning the instrument on and off is also in this location. VIS MODE: TRANSISTOR. To test transistors with the visual indication only, turn the FUNCTION switch to the XSTR-VIS mode. The amber light should flash at about a 1-second rate. Insert the transistor under test in the proper socket. In this mode, you perform two tests on the transistor. The amber light shows the performance of each test. When the amber light is out, this is the EB-BC test mode. When the amber light is on, this is the emitter-collector test mode. The test shows good transistors by one pair of similarly colored lights (green for NPN and red for PNP) when the amber light is off. When the amber light is on, no lights show good transistors. The left-hand lights show the condition of the base-collector. The absence of one or all lights in the EB-BC test mode shows an open or opens. The occurrence of both a red and a green light on either side in the EB-BC test mode shows a short. For more information about the Model 900 tracker, refer to the Maintenance Manual, All Levels for Automatic Transistor Analyzer Model 900, ST810-AD-0PI-010, for patterns other than those just discussed. There are 96 possible patterns listed, VIS MODE: DIODE. You cannot properly test diodes in the XSTER mode. To test a diode, insert the diode in the proper socket and turn the FUNCTION switch to the DIODE/VIS mode. If the diode is good, a pair of green lights will flash out of phase with the amber. If a pair of red lights flash out of phase with the amber light, the diode is either installed improperly or marked improperly. If the diode has a short, additional lights will flash out of phase with the amber. No lights will flash in phase with the amber. You cannot properly test transistors in the DIODE mode. When testing transistors, only one transistor should be in the test socket at one time. Do not leave any diodes in the diode socket while testing transistors. When testing diodes, do not leave transistors in the transistors sockets. If you do not observe these precautions and the devices left in the socket are defective, incorrect light indications will occur. These indications may mislead the operator into believing the device under test is defective. WARNING Unit being tested must be disconnected from ac outlet, and all capacitors capable of storing electricity should be discharged. IN-CIRCUIT TESTING. When testing diodes in-circuit, attach the emitter lead to the anode of the diode. Attach the collector lead to the cathode. When testing transistors, attach the leads to the right terminals as shown by the schematic. If the operator happens to fasten the leads to the transistor in the wrong order, an erroneous display will result. However, if the transistor is good, the instrument will give a good indication. The indication will be for the transistor of the opposite type. A good NPN improperly connected will give good PNP indications and vice versa. If the device is bad, the instrument will give a bad indication. You cannot make a qualitative analysis of the kind of failure unless you attach the proper leads to the correct terminals. To ensure the instrument will show the correct type of transistor (PNP or NPN), you must identify the base lead. Use the following procedure to identify the base lead: 1. Disconnect the lead to the emitter terminal on the instrument. Only the light representing the emitter junction should go out. 2. Reconnect the emitter lead. 8-31

32 Disconnect the lead to the collector terminal of the instrument. Only the light representing the collector junction should go out. Should both lights go out during the tests, the connections are incorrect. Rearrange the leads on the transistor and perform-the tests again. You should now see the proper results. There are six possible combinations for the connection of these leads. Four of these combinations are incorrect. These will cause the instrument to give an incorrect indication as to transistor type (PNP or NPN). The other two combinations will give proper indications, but you still may not know which leads are the emitter and collector terminals. You will know whether the transistor is good and whether it is an NPN or a PNP transistor. If you must know which leads are the emitter and collector terminals, it is possible to find out after identifying the base lead using the meter mode for Beta. SND Mode. In either the XSTR/SND mode or DIODE/SND mode, light patterns showing good devices will have an accompanying beeping sound from the Sonalert. The beeping will be out of phase with the amber light. METER Mode. Before testing a transistor in any of the METER modes, you should test the transistor in one of the visual modes. This will tell you whether the transistor is an NPN or a PNP. After determining this, put the POLARITY switch in the proper position to agree with the indication in the visual mode. Beta. To test the Beta of the transistor, set the FUNCTION switch to the BETA position. Next, set the RANGE switch to the appropriate position according to the power capability of the transistor under test. After the RANGE switch is in the proper position, operate the push-button switch marked ZERO. Now adjust the ADJ knob for a zero reading on the meter. Next, actuate the push-button switch marked BETA and adjust the CAL knob for full-scale deflection, Release the BETA push-button switch; now the Beta of the device will show on the meter. Take care in selecting the Beta range to test the transistor. It is possible to damage small signal transistors should you try to test them in the 2 ma Ib (LG. PWR. XSTR) mode. Leakage: or To test a transistor for or set the FUNCTION switch on the proper position. Next, set the RANGE switch to the 100 ma position. Then push the switch marked ZERO and adjust the ADJ knob for a zero reading on the meter. Now release the ZERO button. Set the RANGE switch on the lowest leakage range, which will still permit less than full-scale deflection on the meter. You may now read the leakage directly off the meter. Read the first and then Use this order because the meter will read down scale when switching from to Also, you can increase the meter sensitivity. However, if you read first and then switch to the meter will read up scale. It is now possible to peg the meter. Although the meter has protection, avoid undue abuse. Material Identity: Transistor. To use this instrument in the IDENTITY mode, set the FUNCTION switch to IDENT. Check the ZERO ADJUST on the meter as mentioned before. After setting the ZERO, release the ZERO push button. Now note whether the needle reads above or below the mark on the meter face just short of half scale, If the meter reads below the mark, the device is a germanium transistor. If it reads above the mark, it is a silicon transistor. This information can be extremely useful when trying to substitute transistors. Leakage: Diode. To test the reverse leakage of diodes, install the diode in the diode socket. You now determine whether the diode is good by testing the device in the visual mode. Once you determine that the diode is good, place the POLARITY switch to NPN. Turn the FUNC- TION switch to the mode, and set the RANGE switch to 100 ma. Now check to see that the meter is at zero, as mentioned before. After zeroing the meter, set the RANGE switch on the lowest range possible that still permits less than full-scale deflection on the meter. Read the leakage on this range. Material Identity: Diode. To test the material identity of a diode with the diode properly installed in the socket, place the POLARITY switch in the PNP position (zero the meter) and the FUNCTION switch in the IDENT position. Using the leads, short the base and collector terminals together. The meter will show either germanium or silicon as described before in the IDENT mode for transistors. 8-32

33 CAUTION Do not identity test transistor material with the base and collector leads shorted together. This may create an erroneous reading. Model 109 Probe The Model 109 probe, used with the Model 900 tester, is easy to use, having one-hand operation. It automatically adjusts to any spacing between one-thirty second inch to five-eighths inch. You can rotate each probe point in a full 360-degree circle. The points are individually spring loaded for proper contact. You can connect the probe to three printed circuit board terminations. The probe has the extremely low contact resistance of less than.005 ohm. The use of the probe eliminates unsoldering while making in-circuit tests of transistors, diodes, ICs, and other components. Finally, the retractable cord stretches to a full 12 feet. DESCRIPTION. The Model 109 three-point probe speeds servicing of printed circuit assemblies that have transistors, diodes, and most other board-mounted components. You can make instant connections to three points on a printed circuit board. You will make rapid evaluation of transistors using the Model 109 probe with the Model 900 automatic transistor analyzer in-circuit. You can accomplish a complete test of all stages in a piece of electronic equipment in a matter of minutes. You can also use the Model 109 to make temporary component substitutions on the printed circuit board. OPERATION. Connect the leads of the Model 109 probe to an appropriate piece of test equipment. Determine the connection points on the printed circuit board to connect to the test equipment. Apply the Model 109 probe points to the circuit board. Press the probe toward the board to ensure a good connection. The Model 109 probe green point is slightly shorter than the yellow and blue probe points. This allows connection of the collector and emitter before the base to provide maximum ease of use. The Model 109 probe is a valuable aid when making resistance and voltage measurements using a conventional VOM or VTVM. Use the yellow and blue probe points as the negative and positive meter feeds. You can make rapid evaluations of entire circuits faster than with any other method because each point pierces through conventional resist coatings and solder residues. Q21. Q22. Q23. Q24. Q25. The Huntron Trackers 1000 and 2000 are for use on circuit boards and systems with all voltage sources in what condition? What mode on the automatic transistor analyzer Model 900 has the Sonalert? What type signal display does the Huntron Trackers 1000 and 2000 show when the signal fuse is open and the test leads shorted together? When using the Huntron Tracker 2000, why must you make good contact with the test leads? What is the minimum total shunt impedance across the junction of the diode or transistor under test using the automatic transistor analyzer Model 900 to ensure a good test reading? SIGNAL GENERATORS Learning Objective: Recognize characteristics and identify the uses of signal generators to include frequency-modulated and pulse-modulated signal generators. Standard sources of RF energy are used to maintain airborne electronic equipment. These energy sources are called signal generators. The principal function of the signal generator is to produce an alternating voltage of the desired frequency and amplitude. The generated signal may be modulated or unmodulated, depending on the test or measurement in question. When using the signal generator, the output signal couples into the circuit under test. You trace its progress through the equipment by using a high-impedance device such as a VTVM or an oscilloscope. RF SIGNAL GENERATORS Radio-frequency signal generators comprise a rather large and very useful class of test equipment. Because of the extremely wide frequency range in the RF region of the spectrum, many signal generators, with different RF ranges 8-33

34 as well as other instrument refinements, are available. FREQUENCY-MODULATED RF SIGNAL GENERATORS Many types of frequency-modulated (FM) signal generators are available for your use; however, some are used for special applications. The following discussion of FM generators provides basic information that applies to most FM generators. An FM signal is one in which the output frequency varies above and below a center frequency. The overall frequency variation is known as the frequency swing (or deviation). The rate at which this swing recurs is controllable at any audio- or video-frequency rate for which the generator is capable. The frequency change of the output is accomplished by the mechanical variation of either the capacitance or inductance of the oscillator circuit or by the use of a reactance tube connected to the oscillator circuit. In the latter case, changes of the voltage impressed on the grid of the reactance tube change the amount of reactance introduced into the oscillator-tuned circuit. As a result, it causes the output frequency to change. The frequency of the signal on the grid of the reactance tube thereby controls the rate of frequency deviation. The amplitude of the signal voltage controls the amount of the deviation. A sweep generator is a form of an FM signal generator. Its carrier deviation is adjustable by a sweep-width control. The sweep generator differs from the ordinary FM signal generator because it maintains the rate of carrier deviation at a fixed frequency. The voltage used to effect the deviation is either a sine wave or a sawtooth waveform. You use an oscilloscope to observe the patterns formed when the passband of interest is swept by this type of generator. The oscilloscope time base must use (or be synchronized with) the same waveform used to produce the deviation. The horizontal (or time) axis of the pattern represents the instantaneous frequency of the generator output. The vertical axis shows the response characteristic of the circuit under test for each frequency. Sweep generators are widely used for observing the response characteristics and the visual alignment of tuned circuits. The sweep generator is used to check the bandwidth of IF amplifiers used in radar receivers. Deviation of the carrier may occur either electromechanically or electronically. The electromechanical method consists of mechanically varying the capacitance or the inductance of the oscillator tank circuit, causing the frequency to vary accordingly. The electronic method makes use of a reactance-tube modulator. A sweep generator produces patterns containing a considerable number of instantaneous frequencies. Marker signals, which are superimposed on the trace, are introduced. These signals orient passband characteristics (or center frequency) of the circuit under test with respect to frequency. The circuit that produces the marker signals may be an integral part of the instrument, or the marker signals may come from an external source. Most modern frequency-swept signal generators use a reactance-tube method of modulation. Modulation of this type results in greater flexibility. Also, the equipment is lighter and more compact than rotating capacitor equipment. The reactance tube and its associated components are connected across the tank circuit of the oscillator in the signal generator. Often, the ac power line, which provides an excellent oscilloscope-synchronizing medium, couples to the grid of the reactance tube to control the rate 8-34

35 of the sweep. The reactance-tube modulator has an advantage over electromechanical modulators because it can be excited by an external variable AF signal generator. The electromechanical modulator is usually limited to single-frequency operations. PULSE-MODULATED RF SIGNAL GENERATORS A pulse-modulated (PM) RF signal generator is similar to the conventional RF signal generator. It differs in its output, which consists of RF energy in the form of pulses that occur at an audio rate. The generator controls can vary the pulsewidth (duration of each pulse) and the repetition rate (number of pulses per second). The PM generator is commonly used to check receiver performance of many radar systems that have a pulse-type emission. A conventional oscillator circuit generates a constant RF carrier to produce pulse-modulated RF signals. This energy goes to the grid of a mixer stage, which has at the same time impressed on its suppressor grid a square wave generated in a separate circuit. The positive half-cycles of the square wave allow the mixer tube to conduct, and the negative half-cycles cut the tube off. During the conducting intervals, the RF signal on the control grid varies the plate current. Therefore, pulses of RF current, corresponding to the positive half-cycles of the square wave, appear in the mixer plate circuit. The pulses normally go to one or more amplifier stages. Controls in the square wave circuit vary pulse time and repetition rate. The Model 628A SHF signal generator (fig. 8-25) is a general-purpose broadband signal generator that produces RF output voltages from 15 GHz to 21 GHz. A single control determines the output frequency, which is directly read on a dial calibrated to an accuracy of ±1 percent or better. The 628A signal generator has some versatile modulation characteristics. It is possible to frequency modulate, square-wave modulate, or pulse modulate the output by internally or externally generated signals. The 628A also provides synchronizing pulses for use with external equipment. Figure Model 628A SHF signal generator front panel. 8-35

36 In addition to producing an accurate and controllable RF signal, you can use the 628A generator to signal Q26. Q27. Q28. Q29. test pulse systems, measure sensitivity and selectivity of amplifiers, receivers, and other tuned systems, measure signal-to-noise ratio of RF signals, make slotted line measurements, investigate microwave impedances and other transmission line characteristics, measure frequency response of microwave systems, and determine resonant frequency and Q of waveguide cavities. What is the principal function of the signal generator? While various types of FM signal generators are available, many are restricted to special applications. What type is used for general applications? Most frequency-swept signal generators use a reactance-tube method of modulation. What is the reason for this? What is a common application for pulsemodulated generators? SIGNAL ANALYZERS Learning Objective: Identify signal analyzers to include signal analysis and waveform measurements including O scope, synchroscope, spectrum analyzers, and distortion analyzers. Signal analyzers, while used in many different situations, are normally used for one purpose to check the response of an equipment under simulated conditions of specific operations. WAVEFORM MEASUREMENT Waveform measurements are made by observing displays of voltage and current variations with respect to time or by harmonic analysis of complex signals. Waveform displays are particularly valuable for adjusting and testing pulse-generator, pulse-former, and pulse-amplifier circuits. The waveform visual display is also useful for determining signal distortion, phase shift, modulation factor, frequency, and peak-to-peak voltage. You can use harmonic analysis test sets to determine the energy distribution in electrical signals. Frequency-selective circuits separate the signals into narrow frequency bands. The energy in each band is indicated by a meter or displayed on a CRT. By connecting a group of frequencyselective circuits in parallel, you can manually or automatically tune a single frequency-selective circuit. You can also use a heterodyne method (using a sweep generator and fixed-tuned circuit) to select electrical power present in a narrow frequency band. OSCILLOSCOPE An oscilloscope or O scope is an electronic test set that displays information on the face of its CRT. There are many ways you can use an oscilloscope; however, its primary use is in troubleshooting and aligning electronic equipment. You do this by observing and analyzing waveform shape, amplitude, and duration. The maintenance instruction manual (MIM) for the particular equipment specifies the waveforms that you should see at the various test points throughout the equipment. Waveforms at any one selected test point may differ, depending on whether the operation of the equipment is normal or abnormal. Figure 8-26 is a typical display you may see on a cathode-ray oscilloscope. This illustration shows the instantaneous voltage of the wave plotted against time. The elapsed time equates to the horizontal distance (view A), from left to right, across the etched grid (graph) placed over the face of the tube. The amplitude of the wave is the vertical measure (view B) on the graph. The oscilloscope also provides picture changes in quantities other than voltages in electric circuits. If an electric current waveform is of interest, you can usually send the current through a small series resistor and look at the voltage wave across the resistor with the oscilloscope. There are also suitable transducers that change other quantities such as temperature, pressure, speed, and acceleration into voltage for display on the oscilloscope. 8-36

37 4.0 divisions. If using the VOLTS/DIV control at 0.5 volt per division, then the voltage difference between points A and B must be 4.0 x 0.5 = 2.0 volts. Figure Typical waveform display: (A) measurement of elapsed time; (B) measurement of voltage difference. Interpreting the Display As you read this paragraph, look at figure Find the elapsed time between two points on the graph (view A, points A and B). Multiply the horizontal distance between these points in major graduated divisions by the setting of the TIME/DIV (time per division) control. This control sets the horizontal sweep rate of the oscilloscope. The distance between points A and B is 4.5 major divisions. If the TIME/DIV control is set at 100 microseconds per division, then the elapsed time between points A and B is 4.5 x 100 = 450 microseconds. In general, elapsed time = horizontal distance (in divisions) x TIME/DIV setting. If you are using the MULTIPLIER control with the TIME/DIV control, multiply the above result by the setting of the MULTIPLIER. If a MAGNIFIER is in operation, divide the result by the amount of magnification. Again, look at figure To find the voltage difference (view B, points A and B) between any two points on the graph, multiply the vertical distance between these points (in major graduated divisions) by the setting of the VOLTS/DIV control. This control sets the vertical deflection factor, or sensitivity, of the oscilloscope. The vertical distance between points A and B is You can express the quantity called pulse repetition rate (or pulse repetition frequency) for periodic pulses as the number of pulses per unit of time. For example, 10 pulses per second and 50 pulses per microsecond. In using the oscilloscope to measure the frequency or repetition rate of periodic waveforms, you read the horizontal distance in major divisions between corresponding points on two succeeding waves first. This is the horizontal distance occupied by one cycle of the wave. Multiply this by the setting of the TIME/DIV control in seconds, milliseconds, or microseconds. Determine the reciprocal of this product; that is, divide 1 by the product. The result is the desired frequency or repetition rate. Square waves, rather than other forms of waves, are usually used to test equipment. By using square waves, you can see more than just a defect s presence; you can see the nature of the defect. The nature of the defect is suggested by the kind of distortion that occurs on a square wave. By observing the square wave response, you, the technician, can easily tell whether the transmission of low or high frequencies is affected. However, this observation is not so clear with regard to frequency with waves other than square waves. Linear devices that give identical responses to square wave inputs generally give responses similar to each other when other waveforms are input to them. Information Contained in a Square Wave A periodic wave contains the following components: 1. A fundamental wave, which is a sine wave having a frequency equal to the repetition frequency of the square wave. 2. An infinite series of odd harmonics sine waves having frequencies that are equal to whole numbers multiplied by the fundamental frequency. The harmonics must be in phase and in amplitude to the fundamental. 8-37

38 Waveform D of figure 8-27 depicts a periodic rectangular wave (square wave). With the square wave, the only harmonics present are the odd harmonics (those whose frequencies are equal to the fundamental frequency multiplied by odd whole numbers). The strengths of the harmonics vary in inverse proportion to the frequencies of the harmonics, the fifth harmonic being one-fifth as strong as the fundamental, for example. Figure 8-27 suggests a way in which these waves combine to make up a square wave. By looking at the four curves shown in figure 8-27, you can see that 1. curve A is the fundamental sine wave, 2. curve B is the sum of the fundamental and third harmonic, 3. curve C is the sum of the fundamental plus third and fifth harmonics, and 4. waveform D is the ultimate square wave. You can see by looking at figure 8-27 that the first few harmonics combine with the fundamental to provide an approach to an actual square wave. Additional harmonics, of higher frequencies, would cause the leading edge of the wave to rise more rapidly. This will produce a sharper corner between the leading edge and the top of the wave. It would require an infinite range of harmonics to produce a truly vertical leading edge and an actual sharp corner. Although this situation is physically impossible to produce, waves can be generated that are very close to this ideal. (The same considerations apply to the falling edge of the waveform and to the following corner.) You can find information about the amplitude and phase relationships of the higher harmonics within the leading-edge steepness and in the sharpness of the corner. If low-frequency components (fundamental and the first few harmonics) are not present in the proper amounts and in the correct phase relationships, the flat top of the square wave is affected. Refer to figure View A shows the Figure 8-27-Addition of harmonics to a fundamental waveform. Figure Information found in a square wave. 8-38

39 location of the low- and high-frequency low-frequency components have lagging phase information in a square wave. Low-frequency angles and are accentuated. defects appear in the form of slope or general curvature in the top (views B and C). In view B, Figure 8-29 is a block diagram of a typical angles and are attenuated. In view C, the oscilloscope, omitting power supplies. The the low-frequency components have leading phase Oscilloscope Block Diagram Figure Typical oscilloscope block diagram. 8-39

40 waveform (A) is input into the vertical amplifier input. The calibrated VOLTS/DIV control sets the gain of this amplifier. The push-pull outputs (B and C) of the vertical amplifier go through a delay line to the vertical deflection plates of the cat bode-ray tube. The time base generator or sweep generator develops a sawtooth wave (E) that is a horizontal deflection voltage. The rising or positive-going part of this sawtooth, called the runup portion of the wave, is linear. It rises through a given number of volts during each unit of time. This rate of rise is set by the calibrated TIME/DIV control. The sawtooth voltage goes to the time base amplifier. This amplifier supplies two output sawtooth waveforms (G and J) simultaneously one of them positive-going, like the input, and the other negative-going. The positive-going sawtooth goes to the right horizontal deflection plate of the CRT, and the negative-going sawtooth goes to the left deflection plate. As a result, the cathode-ray beam sweeps horizontally to the right through a given number of graduated divisions during each unit of time. The TIME/DIV CONTROL establishes the sweep rate. To maintain a stable display on the CRT screen, each horizontal sweep must start at the same point on the waveform. To accomplish this, a sample of the displayed waveform goes to a trigger circuit, which gives a negative output voltage spike (D) at some selected point on the displayed waveform. This triggering spike starts the rising portion of the time base sawtooth. As far as the display is concerned, then, triggering is synonymous with the starting of the horizontal sweep of the trace at the left side of the grid. The rectangular unblanking wave (F) is derived from the time base generator goes to the grid of the CRT. The duration of the positive part of this rectangular wave corresponds with the duration of the positive-going or rising part of the time base output. The beam is switched on during its left-to-right travel and switched off during its right-to-left retrace. Often, the leading edge of the displayed waveform actuates the trigger circuit. However, it may be desirable to observe this leading edge on the screen and the triggering and unblanking operations require a measurable time (P), often about 0.15 microsecond. To see the leading edge, a delay (Q) of about 0.25 microsecond is introduced by the delay line in the vertical deflection channel. The delay occurs after the point where the sample of the vertical signal is tapped off and fed to the trigger circuit. The purpose of the delay line is to retard the application of the observed waveform to the vertical deflection plates. This occurs until the trigger and time base circuits have had an opportunity to begin the unblanking and horizontal sweep operations. This permits viewing the entire desired waveform even though the leading edge of that waveform was used to trigger the horizontal sweep. If the delay line were not used, only that portion of the waveform following the instant (T) in waveform (B) could be seen. Oscilloscope Probe The input circuit to the vertical amplifier (fig. 8-30) of an oscilloscope can be simulated by a high resistance (R) shunted by a small shunt capacitance (C). In some applications, even this high resistance and small capacitance can produce undesirable loading on the circuit whose waveforms are being examined by means oft he oscilloscope. Loading can cause the oscilloscope presentations to be different from the waveforms that would be present with the oscilloscope disconnected. Use of a passive probe reduces this resistive-capacitive loading on the circuit under investigation. The probe (fig. 8-31) includes a resistor shunted by a capacitor This combination is connected in series with the inner conductor of the cable to the oscilloscope input. The result is that when connecting the probe to the circuit under investigation, a new effective loading capacitance smaller than the original capacitance (C) and a new effective loading resistance larger than the original resistance (R) occurs. Thus, the probe reduces the loading effect of the oscilloscope input circuit on the circuit under investigation. A second effect of the probe is to reduce the amount of signal voltage applied directly to the Figure Oscilloscope vertical amplifier input circuit. 8-40

41 Figure Oscilloscope vertical amplifier using a passive probe input. oscilloscope input connection for a given amount of original signal voltage. This occurs because of the voltage-divider action of and R. This effect is taken into account in the attenuation ratio marked on the probe. Thus, if the probe is a 10 x ATTEN, all oscilloscope voltage indications must be multiplied by 10. If an oscilloscope equipped with a probe is used to look at a square wave, and the probe capacitor is too small, some of the highfrequency components of the square wave are bypassed around the oscilloscope input terminals by the input capacitance (C). Thus, the steepness of the leading edge of the displayed square wave (fig. 8-32, view A) is reduced. If the probe capacitor is adjusted to the correct value, a compensating amount of high-frequency information is bypassed around the probe resistor Figure Effects of probe adjustment. (fig. 8-31). To makeup for the loss through C (fig. 8-31), the leading edge of the displayed square wave is restored to its original steepness (fig. 8-32, view B). If (fig. 8-31) is made too large, the high-frequency response of the circuit is overcompensated and applies too much highfrequency information to the oscilloscope input connection. This results in an overshoot in the displayed waveform (fig. 8-32, view C) that was not present in the original waveform. (fig. 8-31) is adjusted to its correct value by using the probe to display the square wave generated by the voltage calibrator, which is a part of the oscilloscope. Adjustment is made to display a square wave with as flat a top as possible. You must check the probe adjustment whenever you use a probe with an oscilloscope or a plug-in preamplifier. This is especially important if the previous use was with an input capacitance different from that of the instrument to which you are now connecting the probe. NOTE: As indicated in figure 8-31, the attenuation achieved is a result of R as well as Though you may swap probes with other types of oscilloscopes, the calibration may be in error even though the waveform distortion may adjust out. 8-41

42 SYNCHROSCOPE The synchroscope is an adaptation of the oscilloscope. Its normal use is for radar applications. A trace occurs only with an input trigger, as contrasted with the continuous sawtooth sweep provided by the oscilloscope. Synchroscope circuits are similar to oscilloscope circuits, with the exception of the signal and the sweep channels. Figure 8-33 shows these circuits in block diagram form. The signal channel of a typical synchroscope includes an input circuit that is usually in the form of a 72-ohm adjustable-step attenuator. Various degrees of attenuation are available, and the calibrated dial indicates how much attenuation is present. The attenuator makes sure all signals, regardless of amplitude, produce about the same input level to the amplifier section. Following the attenuator is an artificial delay line. This low-pass filter has a cutoff frequency higher than the highest passed frequency and an impedance of 72 ohms. The delay line terminates into a 72-ohm gain control. One purpose of the delay line is to delay presentation of the observed signal. The delay lasts until an undelayed portion of the input signal initiates the sweep trace. Without the delay line, the initial portion of the waveform would not appear on the trace. This would occur because a certain amount of time is necessary for the input signal voltage to rise to the level needed to trigger the sweep circuit. With the delay line in use, the signal does not reach the amplifier until one-half microsecond after the trace starts. As a result, you can see the entire pulse. A secondary purpose of the delay line is to provide, by reflection, a series of accurately spaced pulses suitable for calibration of short time intervals. A switch causes a mismatch in the termination of the delay line, causing the secondary purpose. When a sharp pulse is input into the line, a series of reflections occurs similar to those shown in figure Since the time required for a pulse to travel down the line and back is 1 microsecond, a series of pulses occurring 1 microsecond apart occur. Each successive pulse is smaller because of the losses in the delay line, but enough pulses are visible for most high-speed calibration purposes. The gain control feeds a wideband or video amplifier, which connects to the vertical deflection plates. In addition, an external connection is provided to the vertical plates. The horizontal circuit consists of a sync switch for either internal or external sync, a sync amplifier with a gain control, and a start-stop sweep generator. The sweep generator will not develop a sweep voltage until it receives a pulse of enough amplitude. The duration of the sweep, or sweep speed, is adjustable from a very few microseconds to about 250 microseconds. The sweep generator connects to a conventional horizontal amplifier. Since the trace is triggered by the input signal, the synchroscope may be used to observe nonperiodic pulses; for example, the Figure Typical synchroscope block diagram. 8-42

43 Figure Pulse reflection on a mismatched line. nonperiodic pulses occurring in a radar system with an unstable PRF generator. In later designs, provisions are commonly made for calibration of input voltages and sweep time. Voltage calibration is made by comparing the unknown voltage with a variable-voltage pulse of known value, generated internally. The calibrating pulse is adjusted so it is equal in amplitude to the unknown voltage. You can then read the value from the dial that controls the calibrating pulse. Sweep time calibration occurs with the help of marker pulses produced by accurately adjusted tuned circuits. The marker pulses appear on the trace as a series of bright dots spaced at intervals chosen by the operator. In a typical synchroscope, you may select marker intervals of 0.2, 1, 10, 100, and 500 microseconds, depending on the time duration of the pulse under test. Q30. Q31. Q32. Q33. Signal analyzers can be used in many applications. It is used for what function? What determination can you make by observing the square wave response? Look at figure At what point on a square wave does low- and high-frequency information appear? An oscilloscope probe reduces the loading effect of the O-scope input circuit on the circuit under test. What is the second purpose of the probe? Q34. The synchroscope is an adaption of the oscilloscope. What is the difference of the trace on the synchroscope and oscilloscope? SPECTRUM ANALYZER When a radio-frequency carrier wave is modulated by keying, speech or music, or pulses, the resulting wave contains many frequencies. The original carrier is present, together with two groups of new frequencies (sideband components). One group of sidebands is displaced in frequency below the carrier. The other group is displaced above the carrier. The distribution of these frequencies, when shown on a graph of voltage or power against frequency, is called the spectrum of the wave. A spectrum analyzer is a device used to exhibit the spectrum of modulated waves in the radiofrequency range and the microwave region. In principle, the spectrum analyzer operates by tuning through the frequency region in question, using a narrow band receiver. A cathode-ray oscilloscope usually measures the output of the receiver, and the plot on the screen is a graph of voltage versus frequency. The device is essentially a superheterodyne receiver with a very narrowband intermediate frequency amplifier section. The local oscillator frequency varies between two values at a linear rate. The frequency-control generator governs the frequency of the local oscillator. It also produces the horizontal sweep voltage for the CRT deflection plates. (See 8-43

44 Figure Typical spectrum analyzer block diagram. fig ) As a result, each position of the beam corresponds to a definite frequency value, and the display is a graph in which the X-axis is interpreted in terms of frequency. The output of the receiver detector is amplified and goes to the vertical deflection plates. The beam deflects vertically by an amount proportional to the voltage developed in the detector (and amplifier). The signal for analysis goes into the mixer stage of the receiver. The local oscillator changes in frequency at a linear rate, beating with each of the signal frequency components in succession to form the intermediate frequency of the narrowband amplifier. The output of the IF amplifier is detected, amplified, and applied to the vertical deflection plates. Spectrum analyzers designed for analysis of microwave signals have klystron tubes in the local oscillator stage. Analyzers adapted for lower frequency RF signals use triode oscillators that vary through reactance-tube modulators. Spectrum analyzers are the main tool for studying the output of pulse-radar transmitter tubes, such as magnetrons. In this kind of analysis, unwanted effects, such as frequency 8-44

45 occur in the center of the spectrum. You can then read the frequency of the carrier from the calibration of the trap. For more information about spectrum analyzers, refer to NEETS, module 16. In addition, the EIMB Test Methods and Practices, NAVSHIPS 0967-LP , contains detailed discussions of spectrum analysis techniques. Figure Frequency spectra. modulation of the carrier, are easy to detect. In pure amplitude modulation of a carrier wave by a square pulse, the spectrum is symmetrical about the carrier frequency. Lack of symmetry indicates the presence of frequency modulation. Look at view A of figure It shows a spectrum representing the ideal condition. Views B and C show examples of undesirable magnetron spectra. These forms indicate trouble in the modulator, the tuning system, or in the magnetron tube itself. The best definition of carrier frequency is the center frequency in a symmetrical spectrum (fig. 8-36, view A). Some analyzers use this principle as a means of carrier frequency measurement. A sharply resonant circuit in the receiver acts as a trap to prevent an extremely narrow range of frequencies from appearing in the output of the IF amplifier. The result of its use is a gap that appears in the display, and the gap corresponds to the resonant frequency of the trap. The adjustment of the trap is calibrated in frequency, and the circuit can be adjusted to make the gap Echo BOX The echo box is for use in field testing, troubleshooting, and adjusting pulsed-type radar systems. Although simple in construction and operation, it has many applications. If properly used within its design limitations, the echo box can frequently eliminate the need for a complex test setup and an elaborate step-by-step testing procedure. The echo box uses passive circuitry, which does not require any external power other than the radar set whose signal is under analysis. External power requirement is a critical factor with most other test sets. The echo box is similar in operation to a tuned cavity frequency meter; however, it has different capabilities. The tuned cavity frequency meter can measure the frequency of CW or pulsed RF signals in the microwave range. The echo box, however, has no practical application in the testing or analysis of CW equipment signals. Figure 8-37 indicates the basic functional elements of a typical echo box. Energy from the radar transmitter goes through the directional couplers to the resonant Figure Typical echo box functional circuit. 8-45

46 cavity. When the cavity length is properly adjusted, resonant oscillations are set up by each successive pulse of microwave energy. Maximum amplitude of oscillation occurs when the cavity is tuned precisely to the signal frequency. The crystal diode detects these cavity oscillations and indicates them on the meter as an average dc current. The amplitude of oscillation and the average current reading are proportional to the transmitter power output. Oscillations in the tuned cavity also couple back to the radar set under test, where they are processed as an echo signal. This signal, when viewed on the indicator CRT, permits analysis of the radar pulse and presents an indication of the general operating condition of the radar set. Since energy builds up in the cavity, saturation of the cavity is possible. If saturation does occur, distortion of the waveform and erroneous values of the measurements result. If the directional couplers do not prevent cavity saturation, there must be some additional attenuation. Analysis of the displayed waveform can provide a fairly complete functional analysis of the operational condition of a radar set. Among the most important factors it can determine are frequency and bandwidth, power and frequency spectra, sensitivity, pulsewidth and condition, and recovery time. Analysis of the waveform can also prove helpful in locating the cause of malfunctions within the radar set. You need to remember, however, that the echo box presents only relative (rather than absolute) values of power and sensitivity and only rough values of frequency. These quantities are not as accurate as the corresponding values obtained by using a spectrum analyzer. The primary value of the echo box lies in its regular usage. For maximum benefit, you must compare the values from a given test to corresponding values from a test on a radar set you know is operating properly. In general, however, the echo box is an extremely valuable instrument. When used in a continuing maintenance program, it lets the operator maintain the equipment in peak operating condition. Also, it gives indications of deterioration before actual malfunctions occur. Distortion Analyzer The Hewlett-Packard Model 332A distortion analyzer (fig. 8-38) is a solid-state instrument for measuring distortion and ac voltages. The Model 332A includes a high-impedance AM detector that operates from 500 khz to greater than 65 MHz. Distortion levels of 0.1 percent to 100 percent full scale are measured in seven ranges for any fundamental frequency of 5 Hz to 600 khz. Harmonics are indicated up to 3 MHz. The high sensitivity of these instruments requires only 0.3 V rms for the 100 percent set level reference. The OUTPUT connectors provide a low distortion output for monitoring with an oscilloscope, a true rms voltmeter, or a wave analyzer. The instruments are capable of an isolation voltage of 400 volts above chassis ground. You can also use the transistorized voltmeter contained in the Model 332A separately for general-purpose voltage and gain measurements. The voltmeter has a frequency range of 5 Hz to 3 MHz (20 Hz to 500 khz for the 300 µv range), and a voltage range of 300 µv to 300 V rms full scale. The AM detector is a broadband dc restoring peak detector consisting of a semiconductor diode and filter circuit. AM distortion levels as low as 0.3 percent can be measured on a 3 V to 8 V rms carrier modulated 30 percent in the standard broadcast band. Also, lower than 1 percent distortion can be measured at the same level of the carrier up to 65 MHz. The Model 332A distortion analyzer has two modes of operation the distortion mode and the voltmeter mode. Total harmonic distortion measurements from 5 Hz to 600 khz are possible. The distortion mode can indicate harmonics up to 3 MHz. Distortion measurement accuracy is determined by the overall effect of harmonic frequency measurement accuracy, elimination characteristics, distortion introduced by the instrument, and meter accuracy. In the voltmeter mode, the transistorized voltmeter provides a fullscale sensitivity of 300 µv rms (residual noise <25 µv). The voltmeter frequency range is 5 Hz to 3 MHz (20 Hz to 500 khz on the 300 µv range). The distortion measurement accuracy of the 332A is a result of the sharp elimination characteristic of the rejection amplifier circuit and the low level of distortion introduced by the instrument. The fundamental reject ion is at least 80 db, which is small compared to the distortion introduced by the instrument. Thus, low-level harmonic content in the input signal can be measured accurately. You can use the 332A with a wave analyzer for extremely sensitive (>80 db down in the audio-frequency range) measurements of odd harmonics. 8-46

47 ON switch turns instrument ac power on. Pilot lamp glows when instrument is turned ON. NORM-RF DET switch selects front panel INPUT connectors or rear panel RF INPUT connector. INPUT terminals provide connections for input signals. FUNCTION selector selects mode of operation of the instrument. MECHANICAL ZERO ADJUST mechanically zero-sets meter before turning instrument on. DISTORTION/VOLTMETER indicates distortion level and voltage levels of input signals. SENSITIVITY selector provides 0 to 50 db attenuation of input signal in 10 db steps in SET LEVEL and DISTORTION positions of FUNCTION selector. SENSITIVITY VERNIER control provides fine adjustment of attenuation level selected by SENSITIVITY selector. METER RANGE selector selects full-scale range of meter in percentage, db, and rms volts. FREQUENCY RANGE selector selects frequency range to correspond to fundamental frequency of input signal. COARSE BALANCE control provides coarse adjustment for balancing the Wien bridge circuit. FINE BALANCE control provides a vernier adjustment for balancing the Wien bridge circuit. Frequency vernier control provides fine adjustment of FREQUENCY dial. FREQUENCY dial selects fundamental frequency of input signal. OUTPUT connectors provide means of monitoring the output of the meter circuit with an oscilloscope, a true rms voltmeter, or a wave analyzer. RF INPUT connector provides input connection for AM RF carrier input signal. FUSE provides protection for instrument circuits. LINE VOLTAGE (115 V/230 V) switch sets instrument to operate from 115 V or 230 V ac. AC power connector provides input connections for ac power. BATTERY VOLTAGE (+28 to +50 VDC and 28 to 50 VDC) terminals provide connections for external batteries. Figure Model 332A distortion analyzer front and rear panels. 8-47

48 Q35. Describe what factors a spectrum analyzer exhibits. Q36. Describe the purpose of the echo box. Q37. What limitation should you consider when you use the echo box? REFLECTOMETRY TEST SETS Learning Objectives: Recognize the basic theories of time- and frequency-domain reflectometry. Recognize the characteristics of resistive and reactive loads. Recognize TDR displays and identify range and resolution and the uses of analyzing terminations. Identify the advantages and disadvantages of FDR as compared to TDR testers. Recognize the purpose and use of FDR testers. Reflectometry test sets have many uses. They are primarily used to help the organizational maintenance technician verify and troubleshoot aircraft wiring, transmission lines, waveguides, and antenna systems. However, the intermediate maintenance technician can use reflectometry test sets to verify cable connectors, determine test cable impedances, and troubleshoot test equipment. There are two types of reflectometry test sets currently used by the Navy time-domain reflectometer (TDR) and frequency-domain reflectometer (FDR) testers. TIME-DOMAIN REFLECTOMETRY (TDR) TEST SETS You will use time-domain reflectometer (TDR) test sets to check and troubleshoot aircraft wiring, transmission lines, and antenna systems for shorts, opens, crimps, bad couplings, etc. To do this, you will monitor TDR reflected waveforms. TDRs operate on the same principle as radar; that is, they send pulses of energy into a system to see what, if anything, is reflected. Like standing waves on an antenna line, if nothing is reflected, the impedance of the transmission line is uniform and properly terminated. However, if crimps, opens, bad couplings, and so forth, are present, a discontinuity exists, and in-phase or out-ofphase pulses return to the TDR test set. These reflections occur on its CRT as positive, negative, or simply fast-rising voltages, which show the known causes usually at fault. Impedances greater than 50 ohms appear to the TDR as in phase, while those less than 50 ohms appear out of phase. These are respectively classified (traditionally) as inductive and capacitive faults, which are explained by the basic equation: = where L = inductance, C = capacitance, and Z = impedance. TDR Basics The TDR analysis begins with the insertion of a step or pulse of energy (referred to as the incident signal into a system or cable. Then, at the point of insertion, you see the energy reflected by the system or cable under test. Figure 8-39 shows the typical TDR analysis. The output of the pulse generator is, a step signal with a rise time of about 110 picosecond. This signal (incident signal) goes through a sampling tee to the CRT of the sampling oscilloscope and to the system under test via a termination connector. The equivalent bandwidth of the CRT deflection circuits provides a system rise time of about 140 picosecond. This allows the TDR to give resolution (detect faults) as close as one-half inch apart. The reflected signal from the system under test reenters the TDR test set and returns via the sampling tee to the sampling oscilloscope CRT along with the incident signal. By comparing the magnitude, duration, and shape of the reflected signal, you can determine the nature of the impedance variation in the system under test. RESISTIVE LOADS. With a pure resistive load on the output of the TDR, and a step signal applied, a signal whose amplitude is a function of the resistance (fig. 8-40) appears on the CRT. If the line terminates in its characteristic impedance (fig. 8-40), there is no reflected signal. The signal on the CRT will remain flat. However, if the impedance is greater or less than at the termination then reflections (standing-wave ratio [SWR]) exist. The amplitude of the reflected signal is proportional to the value of If is greater than (50 the reflected signal is in phase with the incident signal, and, when applied to the CRT, the reflected signal adds to the incident signal. If is less than the reflected signal is out of phase with the incident signal. When applied to the CRT, the reflected signal subtracts from the incident signal. The dotted lines in figure 8-40 represent various composite signals (incident ± reflected) that you would observe for various values of The time from the start of the incident (step) signal to the 8-48

49 Figure Typical TDR analysis. Figure Step signal-height variations resulting from different resistive loads. step created by the reflected signal represents twice the distance to the discontinuity; that is, the time it took the incident step to reach the discontinuity and return. Most TDRs are calibrated to read this time in feet or inches to the discontinuity. You should separate the system under test from the TDR test set by 8 inches of 50-ohm cable. This moves the reflections away from the leading edge of the step (start of the incident signal) and prevents overshoot and ringing from appearing on the CRT signal. REACTIVE LOADS. The waveform of reactive loads (fig. 8-41) depends on the time Figure TDR reactive load characteristics (time constant = 1). 8-49

50 constant formed by the load and the 50-ohm source. The series RL network (fig. 8-41, view A) appears as an open the instant the step voltage reaches it. This is because the inductor L offers maximum impedance to the change in current caused by the step voltage. Therefore, the reflected signal is in phase with the step voltage and is additive. This explains the sharp rise in voltage. However, as soon as the inductor saturates, the only opposition to current is resistor R. Since L saturates at a nonlinear rate, the voltage drops at a nonlinear rate from the peak of the spike to the same level as the flat portion of the step voltage. At this time, the only load seen by the line is the 50-ohm resistor, which equals the characteristic impedance of the line. The reflections cease until the next step appears at the termination. Then, the cycle repeats itself. To understand the wave shape shown in figure 8-41, view B, you need to remember that L appears as an open to the fast-rising step voltage the instant it is felt at the termination. However, as the inductor saturates, it offers less and less opposition to current until it completely saturates (0 ohm). Since the inductor is parallel to R, the termination is a short, and the reflected wave is 180 degrees out of phase with the incident wave. Since L saturates at a nonlinear rate, the voltage declines at a nonlinear rate. Views C and D of figure 8-41 show a similar analysis of the transmission lines with the RC terminations. The analysis of these different types of discontinuities explains the usefulness of the TDR. Through proper analysis of the discontinuities, you can determine whether they are resistive, inductive, or capacitive and whether it is in series or parallel with the load. TDR in Practice TDR discontinuities have clear separations in time on the CRT. You can easily see the mismatch caused by a connector even if another bad discontinuity is present elsewhere in the system. By using the analysis explained before, you can establish which connector is troublesome and in what way. Once you determine that a discontinuity appears in a waveform, it is simple to locate it in the system. You can save time by calibrating the system so 1 centimeter on the horizontal axis equals a certain number of feet for the transmission system under test. The limiting factor is the system rise time, and any closely spaced discontinuities will appear as a single discontinuity. The finite rise time also limits the size of the distinguishable reactive impedance response. For example, a small shunt capacity in a 50-ohm system causes the waveform to depart from the ideal response (fig. 8-42). The maximum observable line length is a function of the repetition rate chosen. This rate determines the duration of the pulse after its rise. For example, a 200-kHz repetition rate permits the use of TDR devices with up to 1,000 feet of air dielectric cable or 670 feet of polyethylene dielectric coaxial cable. A system s velocity constant determines the speed at which a wave travels through a transmission system. A wave travels faster through air than through polyethylene. This explains the difference in maximum checkable lengths of coaxial cable using a particular repetition rate on the TDR. The longer the cable, the lower the repetition rate must be. 8-50

51 process, even the best connectors will cause reflections or a varying VSWR. Therefore, expensive connectors do not ensure freedom from unwanted reflections. However, the TDR helps you locate unacceptable connectors by rapidly showing where the mismatches are and how bad they are. The TDR also indicates if these connectors are resistive, capacitive, or inductive and whether series or shunt. Figure 8-43 shows a step being propagated from a section of RG9A/U into a load. The connector on the load and the cable are the general radio type 874. It shows four different cases with varying loads. These cases show how you can analyze the connection and the load by using the TDR. With different connectors and loads, the small mismatches (discontinuities) take on different Figure Small shunt capacity in system degrades ideal response. Range and Resolution Assuming that the total impedance equals 50 ohms, you may measure a resistance between ohm and 100 kilohms. Because the height of the reflection is directly proportional to the resistance, you may determine the resistance by using a precalculated transparent overlay. One common use of the TDR is in analyzing a coaxial cable. The amount of impedance variation that is detectable in a long section of cable is a function of the flatness of the top of the incident step. If this step is flat within ±0.5 percent, it can detect an impedance variation of 0.5 ohm along the cable, corresponding to a 1 percent check on cable impedance. Thus, irregularities in cable makeup resulting from variations in the braiding process or tightness of the insulating jacket show up clearly. Analyzing Terminations A departure from 50 ohms in a termination or cable connector can cause some problems. For example, large reflections in a pulse system or a large voltage standing-wave ratio (VSWR) can occur in a system that carries primarily sinusoidal signals. Because of human errors in the assembly Figure Waveforms resulting from the use of different loads. Horizontal scale 0.4 µsec/cm; vertical scale 0.5 percent/cm. 8-51

52 impedance characteristics and the reflected signals change. This change also appears in the wave shape viewed on the oscilloscope. You can compare these signals with those of a normal system by using an overlay showing the pattern of a normal system. The most convenient method to make precise measurements of cable impedance is to connect a section of air dielectric line (with precisely determined impedance) between the cable and the TDR. The step height through the air dielectric line section sets the 50-ohm level. You note any variations from this level in the test cable and calculate the impedance of the cable (fig. 8-44). In this test, the impedance level of the test line is where (Greek letter rho) is the reflection coefficient of the reflected mismatch, If the change in amplitude shows to be +0.03, then Figure Trace of cable shows construction irregularities and increasing series resistance. complex reactive profile (fig. 8-47). Once you determine the proper profile for a particular antenna, you can detect any improper construction details and determine the proper corrective action. FREQUENCY-DOMAIN REFLECTOMETRY (FDR) TEST SETS Frequency-domain reflectometry (FDR) is a fast, simple, and reliable technique developed to The impedance of a long section of coaxial cable would be exactly if there were no line losses. However, most cables have a small series loss and a negligible shunt loss. This series resistance adds to causing the impedance level (as observed at one end of a cable) to increase when adding longer sections of cable. The slope on the step height that results from the increasing impedance is evident in figure There are other applications in which the TDR method of analysis is effective, including component characteristic analysis, antenna analysis, and aircraft wiring checks. You can place the components in an appropriate jig and use the TDR method to determine their shunt capacity and series inductance (fig. 8-46). Investigation of antennas reveals that the TDR pattern is not simple, but instead presents a Figure Resistor checked for shunt capacity special jig. with Figure Oscillograph of step from air dielectric line into test cable. Figure Scope trace of antenna reactive profile. 8-52

53 locate defects in microwave cables and waveguide systems connecting receivers, transmitters, and antennas. Like the TDR, the FDR tester permits direct readout of cable distance, in feet, to the discontinuity (impedance fault). This system has an impressive record of reliability, reduced service time, and improved service standards. Because the FDR checks cables at their actual operating frequencies, discontinuities outside those frequencies do not affect the test. When measurements indicate a fault, you can precisely determine its location (in terms of distance in feet from the point of test). Therefore, you can make repairs quickly and efficiently. FDR vice TDR Until FDR testers, TDR was used as the primary test of cables; a system that has several limitations. For example, TDR measurements cover a spectrum determined by its pulse characteristics; therefore, it detects all discontinuities, including those outside the operating frequency range, which do not affect a system s operation. With the FDR, however, the analysis is within the actual operating frequency band of the microwave system, which assures proper system performance at the operating frequencies. While the FDR works in waveguides and band-limited systems (including transmission networks that contain filters), the TDR cannot work in such systems. The TDR requires a transmission line that passes the whole spectrum from the fundamental frequency (2 MHz to 5 MHz) to the highest harmonic (15 GHz). Waveguides that act as high-pass filters cannot transmit TDR pulses. Similarly, the TDR cannot see through low-pass or bandpass filters because they eliminate the low-frequency harmonics and appear to display a discontinuity on the TDR s CRT. FDR Testing The FDR identifies defective systems by injecting an RF signal into a system and using insertion-loss (attenuation in the line) and returnloss (VSWR) measurements. These measurements help to classify the system under test as good or in need of repair. There are various test setup configurations to measure these losses, based on the particular FDR equipment. Figure 8-48 Figure Typical setup for VSWR and insertion performance. 8-53

54 represents a typical test setup for VSWR and insertion-loss monitoring. Such a test configuration provides simultaneous measurement of the losses. If the input and output connectors of the device under test are accessible, an insertion-loss check verifies input to output performance across the band. For insertion-loss measurement, the network analyzer (fig. 8-48) (using its B and REF channels) indicates the ratio of output signal to input signal directly in db. For tests of long cables whose ends are accessible, the FDR allows measurements from a connector end as far as 2,000 feet from the tester. In some tested systems, however, either the input or output connector may be inaccessible. For such systems, a return-loss measurement made on the accessible connector provides a total system check. For return-loss measurements, the network analyzer (using the A and REF channels) indicates (measures) the ratio of reflected power to incident power directly in db. Incident power is the output of the RF sweep oscillator unit. Figure 8-48 shows how the signals in each case are sampled via directional couplers. Comparison of each measured signal with the incident power of the RF oscillator supplies automatic compensation for any swept-source power variations across the band. This gives a true graph of performance in db versus frequency on the network analyzer CRT. Figure 8-49 shows an example of insertion-loss measurement on the network analyzer CRT. In this example, a loss of less than 10 db is acceptable (as determined from previous tests of a good system). The cable, however, needs repair because a fault (discontinuity) is present, which produces an insertion loss greater than 35 db at a frequency of 3.56 GHz. Figure 8-50 shows a return-loss measurement for the same cable. Here, a loss of 11 db (as determined from a good system), which corresponds to a VSWR of 1.8, is acceptable. At 3.56 GHz, however, the return loss on the CRT indicates 5 db, which corresponds to a VSWR of 3.6, and it is unacceptable. The dual-channel network analyzer in figure 8-48 permits the display of both measurements simultaneously, and both verify the discontinuity in the system cable under test. Single-channel FDR testers require individual test setups for measuring insertion and return losses and comparison of the individual graphs. DETERMINING CABLE LENGTHS OR DISTANCE TO FAULTS. To determine cable length or fault (discontinuity) location measurements (fig. 8-51), a waveguide or a coaxial tee is added in the test setup. You then calibrate the FDR test setup with a calibration cable (provided with FDR set) to establish a known 0-foot reference on the CRT display, Then connect the system cable to the tee. The resulting CRT display of the network analyzer consists of a stationary pattern containing a series of half-dome ripples. A count of the total number of these ripples indicates the number of feet from the cable end to the fault, as shown in figure The FDR display is from the cable that needs repairs (figs and 8-51). Multiply the 5 2/3 ripples by the Figure Insertion-loss display. Figure Return-loss display. 8-54

55 Figure Test setup for fault location measurement. calibration factor of 2 feet per ripple (CRT calibrated that way). You can see that the location of the fault is 11 1/3 feet from the cable end connector (5 2/3 x 2 = 11 1/3 ft). Figure 8-53 shows a dual-channel display of the cable after completing the repairs. The insertion loss is less than 10 db and the return loss is greater than 11 db, indicating proper performance of the system cable. DETAILED FDR ANALYSIS. With the sweep oscillator output, the transmission system under test, and the crystal detector all connected to the same tee junction, discontinuities and/or termination mismatches in the system reflect some of the incident power. The reflected power combines with the incident signal at the crystal detector, resulting in a changing phase relationship that depends on both distance to the discontinuity and signal frequency. As the frequency is swept, it changes the number of wavelengths that occupy the fixed path from the tee to the point of reflection and back. The display Figure Measuring a cable fault. Figure Dual-channel display of a repaired cable. 8-55

56 shows amplitude ripples that result from the summing of the incident and reflected signals. This relationship changes with frequency. Figure 8-54 shows how the magnitude of the vector sum of these signals, which is the signal level detected for display, varies with frequency. The resulting display of the varying-magnitude detected signal is actually a logarithmic SWR presentation. The ripple peaks are adjacent VSWR maxima that occur during the sweep. They occur at each frequency in which the round-trip length of the reflected wave path from the source to the defect has changed by one wavelength. The number of ripples appearing across the full width of the display is a measure of the distance from the discontinuity to the crystal detector. Therefore, a direct readout of fault distance is available when the swept source operates over a sweep width (AF). The sweep width is chosen to provide a display calibration (in terms of ripples per foot) compatible with the length of the transmission system under test. In a coaxial system, the distance to a discontinuity, which may be a fault or the cable end, is represented by the equation Where D is the distance to the fault or cable end in feet, 492 is the half wavelength in feet of a 1-MHz wave in free space transmission, K is the propagation constant that relates the propagation velocity in the coaxial system to the velocity in free space, N is the number of ripples observed in the display, and AF is the swept-frequency excursion (sweep width) of the signal source in MHz. You should note that for any type of cable, AF can be selected to equal 492K. The distance in feet is equal to the number of ripples (including the fractional ripples) shown in the display. Figure Magnitude of the vector sum. 8-56

57 In waveguide systems, the distance down the waveguide to the fault is represented by the same equation, with K as the relation is the wavelength in free space and g is the wavelength in the waveguide) at the frequency of measurement. Describe some of the main uses for the TDR. Describe the basics of TDR. While you can determine different types of discontinuities with the TDR, what else can you determine through proper analysis? What factor determines the speed at which a wave travels through a transmission system? By what method does using a TDR help you locate an unacceptable connector? While TDR and FDR provide similar measurements, the FDR eliminates what limitation of the TDR? Q38. Q39. Q40. Q41. Q42. Q43. Q44. Q45. Describe the means by which the FDR identifies defective systems. When determining cable lengths or distance to faults, what means do you use to determine the number of feet from the cable end to the fault? VAST STATION Learning Objective: Identify features, components, and operating procedures of a typical ATE VAST station. U.S. Navy aircraft carriers and shore installations are equipped with automatic test equipments (ATEs), such as the Versatile Avionics Shop Test (VAST) station, AN/USM-247(V), and the Hybrid Automatic Test System (HATS), AN/USM-403. The VAST and HATS deal with the continually changing field of avionics testing. The use of these computerized ATEs has significantly reduced the space requirements of special- and manual-support test equipments, The discussion contained in this chapter deals with the VAST station. TYPICAL VAST STATION In its basic form, a VAST station is assembled from an inventory of functional building blocks. These building blocks furnish all the necessary stimuli and have the measurement capability to check current naval avionics equipment. As new equipment is developed and introduced, the test station configuration may be modified. As it becomes necessary, new building blocks furnish new parameters or greater precision to existing capabilities. A typical VAST station (fig. 8-55) consists of a computer subsystem, a data transfer unit Figure Typical carrier-based VAST station. 8-57

58 (DTU), and a stimulus and measurement section containing functional building blocks configured to meet the intended test application. A computer subsystem controls the test station, which executes test programs to assure accurate and satisfactory testing. The computer subsystem includes a general-purpose digital computer that executes test routines and has diagnostic and computational capabilities. Also, this subsystem processes data and furnishes a permanent record of test results. Two magnetic tape transports provide rapid access to avionics test programs and immediate availability of VAST self-check programs. The data transfer unit (DTU) (fig. 8-56) serves as the operator-machine interface. It synchronizes instructions and data flow between the computer and the functional building blocks. Also, it contains the display and control panels. The operator communicates with the computer and the stimulus and measurement section of the VAST system by using the DTU control panel, which has the keyboard and mode select key. The test station may be operated in three modes manual, semiautomatic, or fully automatic. The DTU contains a maintenance panel that monitors station auto-check results and indicates building block faults. Transmission of instructions from the control computer is on a request/ acknowledge basis. Essentially, the stimulus and measurement section controls the response rate. This allows instructions to be transmitted at an asynchronous rate, corresponding to the 8-58

59 maximum frequency at which a given building block or avionics unit can respond. Therefore, there is no requirement for immediate program storage in the DTU. FEATURES OF A VAST STATION Figure Data transfer unit (DTU). A VAST station may have as many as 14 racks of stimulus and measurement building blocks (fig. 8-57). Large station configurations may contain as many as 17 core building blocks. Core building blocks are designated as a result of high-use factors or because they are needed for self-test requirements. Building blocks not in the core category are usually selected to meet the specific test requirements of shop operations or avionics equipment on board ship. In general, the location of such peripheral building blocks is flexible. To maintain standardization between VAST stations, the effects of building block interconnection cable losses and switches have to remain within predictable limits; this is the purpose of the core concept. Ease of maintenance is the main objective of the VAST station designed. In addition to the modularized design of VAST building blocks, there are three levels of fault detection, which ensure rapid confidence tests and easy fault location. The three levels of detection are auto-check. self-check, and self-test. Fault detection may be initially made through auto-check. The auto-check is inherent in the logic and control design of the test station and includes Figure VAST station with building blocks. 8-59

60 verification of instructions and fault monitoring. Auto-check is carried out on a continuous basis during station operation and, when a fault occurs, testing is interrupted. The second level of VAST fault detection is self-check. Self-check is a programmed sequence that is initiated by the VAST operator through the DTU keyboard. Self-check may be either internal or at the system level. Internal self-check measures the ability of a building block to perform against its own internal standards. System self-check requires the use of two or more building blocks in a test configuration selected to isolate faults within the test setup. The self-check philosophy used to verify the operation of VAST is based upon confirmation of key system elements first. Then, these elements are used to check the remaining building blocks. Fundamental core building blocks are checked by means of internal standards. Once satisfactory performance is assured, their capabilities are used to check the remaining building blocks, The checkout of noncore building blocks is accomplished by using any combination(s) of core measurement and stimulus building blocks. The final level of VAST fault detection is self-test. This is a series of test programs used to locate faults within a building block. If a building block has been found to contain a malfunction as a result of a self-check routine, then self-test programs are conducted. This is done by removing the faulty building block from the VAST rack and by connecting it to the test station in the same manner as if it were a unit under test. Avionics equipment must be designed to be adaptable to automatic testing to assure optimum support by VAST. Moreover, test programs must be prepared that are compatible with VAST performance characteristics. VAST-TO-UUT INTERCONNECTING DEVICE Included in the program design is the allimportant interconnecting device design. In its simplest form, the interconnecting device consists of an adapter cable, which connects the unit under test (UUT) to the VAST interface. In some cases, however, it is necessary to introduce, as part of the electrical interface in the interconnecting device, passive and active circuits to change impedance levels or to amplify low signals, Ordinarily, this is not required if avionics equipment has been designed within the requirements of VAST. Often, passive circuit functions are obtained through the use of standard plug-in modules. The last element of the test program is the instruction booklet or microfilm strip. This element details all the steps to follow when you test any given unit, from initial procedures, such as hookup and clearing operations, down to the final stages of disconnect and UUT closeout. OPERATION OF A VAST STATION In the typical VAST test procedure, ease of operation in the actual testing becomes apparent, The initial setup of the weapon replaceable assembly, including removal of dust covers, cooling provisions, and connections to interface device, may be made off station to minimize disruptions of station operators. Final connections between the VAST station s interface panel and the UUT are made in a few moments at the station. The operator begins testing by selecting the code that initiates the test program. Before power or stimulus is applied to the UUT, continuity tests are run to make sure the proper test program has been selected and no condition exists that will damage the VAST station or the UUT once active tests are started. If everything checks out, the testing proceeds automatically, The operator only has to respond to instructions that appear on the CRT display. The program will not stop until a fault is encountered or a program halt is reached. The purpose of programmed halts is to allow manual intervention during the course of testing to make adjustments and observations. When the identification of faults and the operator s instructions are required (such as interpreting a complex waveform), the operator may be referred to the test program instructions. Upon completion of the test program, the CRT display indicates closeout procedures. A VAST station is completely autonomous and normally operated under computer control in a fully automatic mode, stopping only as previously mentioned. Of course, the operator can select any one of the semiautomatic modes or a manual mode. The semiautomatic modes include a onegroup, one-test, and one-step mode. These auxiliary modes permit detailed observation of various test sequences, and they are useful 8-60

61 in performing work-around procedures in reconciling differences in equipment and program mode status and in the verification of repairs. In the manual mode, the test station is completely off-line with respect to the computer. Instructions are introduced by the operator through the keyboard on a one-word-at-a-time basis. (See fig ) Although the manual mode is never used for avionics testing, it is useful for debugging new programs, integrating new building blocks into the station, and performing self-check operations on some of the building blocks. Q46. List the elements of a typical VAST station. Q47. List the three levels of detection that ensure rapid confidence tests and easy fault detection. Q48. What is the purpose of programmed halts? Figure Typical VAST control panel. 8-61